Tweak probabilities in random equiv test. (#7992)

Previously, Hypothesis spent most of the test cycles adding
registers rather than useful instructions, and would in
fact skip running the equivalence test rule altogether,
so we'd pretty much expect testing to pass no matter
what.

This commit limits the number of classical and quantum
registers to 3 each (arbitrary) to increase the chances
of gate generation and invocation of the equivalence
transpilation rule.

The number of examples has also been increased from 25
to 200, which should vastly improve the coverage of
this test, at the expense of a longer runtime.

test/randomized/test_transpiler_equivalence.py

@@ -99,7 +99,7 @@ default_profile = "transpiler_equivalence"
 settings.register_profile(
     default_profile,
     report_multiple_bugs=False,
-    max_examples=25,
+    max_examples=200,
     deadline=None,
     suppress_health_check=[HealthCheck.filter_too_much],
 )
@@ -261,12 +261,17 @@ class QCircuitMachine(RuleBasedStateMachine):
     backend = Aer.get_backend("aer_simulator")
     max_qubits = int(backend.configuration().n_qubits / 2)
 
+    # Limit reg generation for more interesting circuits
+    max_qregs = 3
+    max_cregs = 3
+
     def __init__(self):
         super().__init__()
         self.qc = QuantumCircuit()
         self.enable_variadic = bool(variadic_gates)
 
     @precondition(lambda self: len(self.qc.qubits) < self.max_qubits)
+    @precondition(lambda self: len(self.qc.qregs) < self.max_qregs)
     @rule(target=qubits, n=st.integers(min_value=1, max_value=max_qubits))
     def add_qreg(self, n):
         """Adds a new variable sized qreg to the circuit, up to max_qubits."""
@@ -275,6 +280,7 @@ class QCircuitMachine(RuleBasedStateMachine):
         self.qc.add_register(qreg)
         return multiple(*list(qreg))
 
+    @precondition(lambda self: len(self.qc.cregs) < self.max_cregs)
     @rule(target=clbits, n=st.integers(1, 5))
     def add_creg(self, n):
         """Add a new variable sized creg to the circuit."""