Fix parallel test setup

In the previous patches we were attempting to use
unittest.mock.patch.dict to override the env variable used to control
parallel execution in qiskit. However, the issue with doing this is that
the env variable is read at import time and stored in a global variable
as we don't expect the env variable to change dynamically during the
execution of a script. To workaround this the mocks are removed and
instead a setUp() method is added to the test class to override the
whatever the environment default is and instead hardcode parallel_map to
run in parallel for the test and then switch it back to the earlier
value after the test finishes. This lets us dynamically adjust the
default behavior for parallel execution for this test class.

test/python/compiler/test_transpiler.py

@@ -60,6 +60,7 @@
 from qiskit.quantum_info import Operator, random_unitary
 from qiskit.transpiler.passmanager_config import PassManagerConfig
 from qiskit.transpiler.preset_passmanagers import level_0_pass_manager
+from qiskit.tools import parallel
 
 
 class CustomCX(Gate):
@@ -1795,6 +1796,18 @@ def test_custom_multiple_circuits(self):
 class TestTranspileParallel(QiskitTestCase):
     """Test transpile() in parallel."""
 
+    def setUp(self):
+        super().setUp()
+
+        # Force parallel execution to True to test multiprocessing for this class
+        original_val = parallel.PARALLEL_DEFAULT
+
+        def restore_default():
+            parallel.PARALLEL_DEFAULT = original_val
+
+        self.addCleanup(restore_default)
+        parallel.PARALLEL_DEFAULT = True
+
     @data(0, 1, 2, 3)
     def test_parallel_with_target(self, opt_level):
         """Test that parallel dispatch works with a manual target."""
@@ -1803,8 +1816,7 @@ def test_parallel_with_target(self, opt_level):
         qc.cx(0, 1)
         qc.measure_all()
         target = FakeMumbaiV2().target
-        with patch.dict("os.environ", {"QISKIT_PARALLEL": "TRUE"}):
-            res = transpile([qc] * 3, target=target, optimization_level=opt_level)
+        res = transpile([qc] * 3, target=target, optimization_level=opt_level)
         self.assertIsInstance(res, list)
         for circ in res:
             self.assertIsInstance(circ, QuantumCircuit)