Fix oppia#4097: Adding tests for math utils (oppia#5627)

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## Explanation
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Fix oppia#4097
This PR adds test files for the `FractionSubject`, `RealSubject`,
`MathExpressionSubject`, and `MathEquationSubject` classes and includes
a `BUILD.bazel` file to enable building and running these tests.

## Essential Checklist
<!-- Please tick the relevant boxes by putting an "x" in them. -->
- [x] The PR title and explanation each start with "Fix #bugnum: " (If
this PR fixes part of an issue, prefix the title with "Fix part of
#bugnum: ...".)
- [x] Any changes to
[scripts/assets](https://github.com/oppia/oppia-android/tree/develop/scripts/assets)
files have their rationale included in the PR explanation.
- [x] The PR follows the [style
guide](https://github.com/oppia/oppia-android/wiki/Coding-style-guide).
- [x] The PR does not contain any unnecessary code changes from Android
Studio
([reference](https://github.com/oppia/oppia-android/wiki/Guidance-on-submitting-a-PR#undo-unnecessary-changes)).
- [x] The PR is made from a branch that's **not** called "develop" and
is up-to-date with "develop".
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testing/src/test/java/org/oppia/android/testing/math/BUILD.bazel

@@ -0,0 +1,69 @@
+"""
+Tests for math-related test utilities.
+"""
+
+load("//:oppia_android_test.bzl", "oppia_android_test")
+
+oppia_android_test(
+    name = "FractionSubjectTest",
+    srcs = ["FractionSubjectTest.kt"],
+    custom_package = "org.oppia.android.testing.math",
+    test_class = "org.oppia.android.testing.math.FractionSubjectTest",
+    test_manifest = "//testing:test_manifest",
+    deps = [
+        "//model/src/main/proto:math_java_proto_lite",
+        "//testing/src/main/java/org/oppia/android/testing/math:fraction_subject",
+        "//testing/src/main/java/org/oppia/android/testing/robolectric:test_module",
+        "//third_party:com_google_truth_truth",
+        "//third_party:junit_junit",
+        "//third_party:robolectric_android-all",
+    ],
+)
+
+oppia_android_test(
+    name = "MathEquationSubjectTest",
+    srcs = ["MathEquationSubjectTest.kt"],
+    custom_package = "org.oppia.android.testing.math",
+    test_class = "org.oppia.android.testing.math.MathEquationSubjectTest",
+    test_manifest = "//testing:test_manifest",
+    deps = [
+        "//model/src/main/proto:math_java_proto_lite",
+        "//testing/src/main/java/org/oppia/android/testing/math:math_equation_subject",
+        "//testing/src/main/java/org/oppia/android/testing/robolectric:test_module",
+        "//third_party:com_google_truth_truth",
+        "//third_party:junit_junit",
+        "//third_party:robolectric_android-all",
+    ],
+)
+
+oppia_android_test(
+    name = "MathExpressionSubjectTest",
+    srcs = ["MathExpressionSubjectTest.kt"],
+    custom_package = "org.oppia.android.testing.math",
+    test_class = "org.oppia.android.testing.math.MathExpressionSubjectTest",
+    test_manifest = "//testing:test_manifest",
+    deps = [
+        "//model/src/main/proto:math_java_proto_lite",
+        "//testing/src/main/java/org/oppia/android/testing/math:math_expression_subject",
+        "//testing/src/main/java/org/oppia/android/testing/robolectric:test_module",
+        "//third_party:com_google_truth_truth",
+        "//third_party:junit_junit",
+        "//third_party:robolectric_android-all",
+    ],
+)
+
+oppia_android_test(
+    name = "RealSubjectTest",
+    srcs = ["RealSubjectTest.kt"],
+    custom_package = "org.oppia.android.testing.math",
+    test_class = "org.oppia.android.testing.math.RealSubjectTest",
+    test_manifest = "//testing:test_manifest",
+    deps = [
+        "//model/src/main/proto:math_java_proto_lite",
+        "//testing/src/main/java/org/oppia/android/testing/math:real_subject",
+        "//testing/src/main/java/org/oppia/android/testing/robolectric:test_module",
+        "//third_party:com_google_truth_truth",
+        "//third_party:junit_junit",
+        "//third_party:robolectric_android-all",
+    ],
+)

testing/src/test/java/org/oppia/android/testing/math/FractionSubjectTest.kt

@@ -0,0 +1,206 @@
+package org.oppia.android.testing.math
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+import org.oppia.android.app.model.Fraction
+
+/** Tests for [FractionSubject]. */
+@RunWith(JUnit4::class)
+class FractionSubjectTest {
+
+  @Test
+  fun testHasNegativeProperty_withNegativeFraction_matchesTrue() {
+    val fraction = Fraction.newBuilder()
+      .setIsNegative(true)
+      .build()
+
+    FractionSubject.assertThat(fraction).hasNegativePropertyThat().isTrue()
+  }
+
+  @Test
+  fun testHasNegativeProperty_withPositiveFraction_matchesFalse() {
+    val fraction = Fraction.newBuilder()
+      .setIsNegative(false)
+      .build()
+
+    FractionSubject.assertThat(fraction).hasNegativePropertyThat().isFalse()
+  }
+
+  @Test
+  fun testHasNegativeProperty_defaultValue_matchesFalse() {
+    val fraction = Fraction.newBuilder().build()
+
+    FractionSubject.assertThat(fraction).hasNegativePropertyThat().isFalse()
+  }
+
+  @Test
+  fun testHasWholeNumber_withPositiveValue_matchesValue() {
+    val fraction = Fraction.newBuilder()
+      .setWholeNumber(5)
+      .build()
+
+    FractionSubject.assertThat(fraction).hasWholeNumberThat().isEqualTo(5)
+  }
+
+  @Test
+  fun testHasWholeNumber_withMaxUint32_matchesValue() {
+    val fraction = Fraction.newBuilder()
+      .setWholeNumber(Int.MAX_VALUE)
+      .build()
+
+    FractionSubject.assertThat(fraction).hasWholeNumberThat().isEqualTo(Int.MAX_VALUE)
+  }
+
+  @Test
+  fun testHasWholeNumber_defaultValue_matchesZero() {
+    val fraction = Fraction.newBuilder().build()
+
+    FractionSubject.assertThat(fraction).hasWholeNumberThat().isEqualTo(0)
+  }
+
+  @Test
+  fun testHasNumerator_withPositiveValue_matchesValue() {
+    val fraction = Fraction.newBuilder()
+      .setNumerator(3)
+      .build()
+
+    FractionSubject.assertThat(fraction).hasNumeratorThat().isEqualTo(3)
+  }
+
+  @Test
+  fun testHasNumerator_withMaxUint32_matchesValue() {
+    val fraction = Fraction.newBuilder()
+      .setNumerator(Int.MAX_VALUE)
+      .build()
+
+    FractionSubject.assertThat(fraction).hasNumeratorThat().isEqualTo(Int.MAX_VALUE)
+  }
+
+  @Test
+  fun testHasNumerator_defaultValue_matchesZero() {
+    val fraction = Fraction.newBuilder().build()
+
+    FractionSubject.assertThat(fraction).hasNumeratorThat().isEqualTo(0)
+  }
+
+  @Test
+  fun testHasDenominator_withPositiveValue_matchesValue() {
+    val fraction = Fraction.newBuilder()
+      .setDenominator(4)
+      .build()
+
+    FractionSubject.assertThat(fraction).hasDenominatorThat().isEqualTo(4)
+  }
+
+  @Test
+  fun testHasDenominator_withMaxUint32_matchesValue() {
+    val fraction = Fraction.newBuilder()
+      .setDenominator(Int.MAX_VALUE)
+      .build()
+
+    FractionSubject.assertThat(fraction).hasDenominatorThat().isEqualTo(Int.MAX_VALUE)
+  }
+
+  @Test
+  fun testHasDenominator_defaultValue_matchesZero() {
+    val fraction = Fraction.newBuilder().build()
+
+    FractionSubject.assertThat(fraction).hasDenominatorThat().isEqualTo(0)
+  }
+
+  @Test
+  fun testEvaluatesToDouble_withProperFraction_matchesExpectedValue() {
+    val fraction = Fraction.newBuilder()
+      .setNumerator(3)
+      .setDenominator(4)
+      .build()
+
+    FractionSubject.assertThat(fraction).evaluatesToDoubleThat().isEqualTo(0.75)
+  }
+
+  @Test
+  fun testEvaluatesToDouble_withImproperFraction_matchesExpectedValue() {
+    val fraction = Fraction.newBuilder()
+      .setNumerator(5)
+      .setDenominator(2)
+      .build()
+
+    FractionSubject.assertThat(fraction).evaluatesToDoubleThat().isEqualTo(2.5)
+  }
+
+  @Test
+  fun testEvaluatesToDouble_withMixedNumber_matchesExpectedValue() {
+    val fraction = Fraction.newBuilder()
+      .setWholeNumber(2)
+      .setNumerator(3)
+      .setDenominator(4)
+      .build()
+
+    FractionSubject.assertThat(fraction).evaluatesToDoubleThat().isEqualTo(2.75)
+  }
+
+  @Test
+  fun testEvaluatesToDouble_withNegativeValue_matchesExpectedValue() {
+    val fraction = Fraction.newBuilder()
+      .setIsNegative(true)
+      .setWholeNumber(2)
+      .setNumerator(1)
+      .setDenominator(2)
+      .build()
+
+    FractionSubject.assertThat(fraction).evaluatesToDoubleThat().isEqualTo(-2.5)
+  }
+
+  @Test
+  fun testProtoEquality_withIdenticalValues_areEqual() {
+    val fraction1 = Fraction.newBuilder()
+      .setIsNegative(true)
+      .setWholeNumber(3)
+      .setNumerator(2)
+      .setDenominator(5)
+      .build()
+
+    val fraction2 = Fraction.newBuilder()
+      .setIsNegative(true)
+      .setWholeNumber(3)
+      .setNumerator(2)
+      .setDenominator(5)
+      .build()
+
+    assertThat(fraction1).isEqualTo(fraction2)
+  }
+
+  @Test
+  fun testProtoSerialization_withComplexFraction_maintainsValues() {
+    val originalFraction = Fraction.newBuilder()
+      .setIsNegative(true)
+      .setWholeNumber(3)
+      .setNumerator(2)
+      .setDenominator(5)
+      .build()
+
+    val bytes = originalFraction.toByteArray()
+    val deserializedFraction = Fraction.parseFrom(bytes)
+
+    FractionSubject.assertThat(deserializedFraction).apply {
+      hasNegativePropertyThat().isTrue()
+      hasWholeNumberThat().isEqualTo(3)
+      hasNumeratorThat().isEqualTo(2)
+      hasDenominatorThat().isEqualTo(5)
+    }
+  }
+
+  @Test
+  fun testDefaultInstance_hasDefaultValues() {
+    val defaultFraction = Fraction.getDefaultInstance()
+
+    FractionSubject.assertThat(defaultFraction).apply {
+      hasNegativePropertyThat().isFalse()
+      hasWholeNumberThat().isEqualTo(0)
+      hasNumeratorThat().isEqualTo(0)
+      hasDenominatorThat().isEqualTo(0)
+    }
+  }
+}