Change RB test parameters to make the test faster and more robust (#1295

)

`test_add_more_circuit_yields_lower_variance` started failing with
qiskit-aer 0.13.0. The test was only comparing the results of 3 RB
sampels to 5 RB samples and looking for the case with more samples to
have lower variance. These numbers of samples were likely not enough to
ensure that the case with more samples always had lower variance across
a wide range of random seeds. Here the high number of samples was
increased to 30.

To make sure that the fit results were reasonable and that the test run
time did not get too long, these additional changes were:

* Convert the test from two qubit RB to single qubit RB. This makes the
simulation faster.
* Increase the single qubit gate error. This makes the qubit depolarize
faster so that the RB curve can be traced out with less gates.

test/library/randomized_benchmarking/test_standard_rb.py

@@ -319,11 +319,29 @@ def test_three_qubit(self):
 
     def test_add_more_circuit_yields_lower_variance(self):
         """Test variance reduction with larger number of sampling."""
+
+        # Increase single qubit error so that we can see gate error with a
+        # small number of Cliffords since we want to run many samples without
+        # taking too long.
+        p1q = 0.15
+        pvz = 0.0
+
+        # setup noise model
+        sx_error = depolarizing_error(p1q, 1)
+        rz_error = depolarizing_error(pvz, 1)
+
+        noise_model = NoiseModel()
+        noise_model.add_all_qubit_quantum_error(sx_error, "sx")
+        noise_model.add_all_qubit_quantum_error(rz_error, "rz")
+
+        # Aer simulator
+        backend = AerSimulator(noise_model=noise_model, seed_simulator=123)
+
         exp1 = rb.StandardRB(
-            physical_qubits=(0, 1),
+            physical_qubits=(0,),
             lengths=list(range(1, 30, 3)),
             seed=123,
-            backend=self.backend,
+            backend=backend,
             num_samples=3,
         )
         exp1.analysis.set_options(gate_error_ratio=None)
@@ -332,11 +350,11 @@ def test_add_more_circuit_yields_lower_variance(self):
         self.assertExperimentDone(expdata1)
 
         exp2 = rb.StandardRB(
-            physical_qubits=(0, 1),
+            physical_qubits=(0,),
             lengths=list(range(1, 30, 3)),
             seed=456,
-            backend=self.backend,
-            num_samples=5,
+            backend=backend,
+            num_samples=30,
         )
         exp2.analysis.set_options(gate_error_ratio=None)
         exp2.set_transpile_options(**self.transpiler_options)