Adding test coverage for patter template override

tests/modules/ui_patterns_render_test/templates/pattern-foo-bar.html.twig

@@ -1 +1 @@
-Foo Bar
+Foo Bar default template

tests/src/Functional/UiPatternsPreviewRenderTest.php

@@ -59,6 +59,19 @@ public function testPatternPreviewSuggestions() {
     foreach ($suggestions as $suggestion) {
       $assert_session->responseContains($suggestion);
     }
+
+    // Pattern foo-bar default template is loaded.
+    $assert_session->pageTextContains('Foo Bar default template');
+    $assert_session->pageTextNotContains('Foo Bar overridden template');
+
+    // Install test theme to load template(s) overrides.
+    $this->container->get('theme_installer')->install(['ui_patterns_theme_test']);
+    $this->container->get('theme_handler')->setDefault('ui_patterns_theme_test');
+    $this->container->set('theme.registry', NULL);
+
+    $this->drupalGet('/patterns');
+    $assert_session->pageTextContains('Foo Bar overridden template');
+    $assert_session->pageTextNotContains('Foo Bar default template');
   }
 
 }

tests/themes/ui_patterns_theme_test/pattern-foo-bar--variant-default--preview.html.twig

@@ -0,0 +1 @@
+Foo Bar overridden template

tests/themes/ui_patterns_theme_test/ui_patterns_theme_test.info.yml

@@ -0,0 +1,4 @@
+name: 'UI Patterns Test theme'
+type: theme
+description: 'Theme for testing UI Patterns features in action on a theme.'
+core: 8.x