Also copy `__module__` to our dynamically created TestCase subclass when...

Also copy `__module__` to our dynamically created TestCase subclass when override_settings acts as a class decorator.

Otherwise, an error in e.g. a test in tests/modeltests/proxy_model_inheritance/tests.py
was being reported as:

ERROR: test_table_exists (django.test.utils.ProxyModelInheritanceTests)

Refs #16672 and [16650].

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16722 bcc190cf-cafb-0310-a4f2-bffc1f526a37