Fix flakiness in pulse-optimal UnitarySynthesis test (#11307)

The previous iteration of this test asserted that the `sx` count for
non-optimal synthesis was higher than a certain particular value.  This
value did not have any fundamental properties, it was just the value
that happened to be returned for some time.  Recent OpenBLAS support
for x86_64 instructions from the AVX512 SkylakeX set meant that
supporting processors can now return slightly fewer `sx` gates in the
non-optimal path, despite the pulse-optimal synthesis still not being in
use.  This caused flaky CI, when we were allocated an Azure VM that had
access to the new instructions.

(cherry picked from commit c7ecb5f)

test/python/transpiler/test_unitary_synthesis.py

@@ -344,25 +344,36 @@ def test_two_qubit_synthesis_not_pulse_optimal(self):
         backend = FakeVigo()
         conf = backend.configuration()
         qr = QuantumRegister(2)
-        coupling_map = CouplingMap([[0, 1], [1, 2], [1, 3], [3, 4]])
-        triv_layout_pass = TrivialLayout(coupling_map)
         qc = QuantumCircuit(qr)
         qc.unitary(random_unitary(4, seed=12), [0, 1])
-        unisynth_pass = UnitarySynthesis(
-            basis_gates=conf.basis_gates,
-            coupling_map=coupling_map,
-            backend_props=backend.properties(),
-            pulse_optimize=False,
-            natural_direction=True,
+        coupling_map = CouplingMap([[0, 1]])
+        pm_nonoptimal = PassManager(
+            [
+                TrivialLayout(coupling_map),
+                UnitarySynthesis(
+                    basis_gates=conf.basis_gates,
+                    coupling_map=coupling_map,
+                    backend_props=backend.properties(),
+                    pulse_optimize=False,
+                    natural_direction=True,
+                ),
+            ]
         )
-        pm = PassManager([triv_layout_pass, unisynth_pass])
-        qc_out = pm.run(qc)
-        if isinstance(qc_out, QuantumCircuit):
-            num_ops = qc_out.count_ops()
-        else:
-            num_ops = qc_out[0].count_ops()
-        self.assertIn("sx", num_ops)
-        self.assertGreaterEqual(num_ops["sx"], 16)
+        pm_optimal = PassManager(
+            [
+                TrivialLayout(coupling_map),
+                UnitarySynthesis(
+                    basis_gates=conf.basis_gates,
+                    coupling_map=coupling_map,
+                    backend_props=backend.properties(),
+                    pulse_optimize=True,
+                    natural_direction=True,
+                ),
+            ]
+        )
+        qc_nonoptimal = pm_nonoptimal.run(qc)
+        qc_optimal = pm_optimal.run(qc)
+        self.assertGreater(qc_nonoptimal.count_ops()["sx"], qc_optimal.count_ops()["sx"])
 
     def test_two_qubit_pulse_optimal_true_raises(self):
         """Verify raises if pulse optimal==True but cx is not in the backend basis."""