If a custom target was specified when calling the transpile() function
with more than one circuit the parallel dispatch of the shared custom
target would fail because the basis_gates created from the target was a
dict keys type. Pickle is unable to serialize a dict keys object
apparently and this would cause an exception to be raised. This commit
fixes this by casting the basis_gates from the target as a list so that
it can be serialized.

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.