Fixed #15368 -- Ensure that raw unittest TestCase instances can be invoked...

import unittest as real_unittest

from django.conf import settings

from django.core.exceptions import ImproperlyConfigured

from django.db.models import get_app, get_apps

            TestClass = getattr(test_module, parts[1], None)

    try:

        if issubclass(TestClass, unittest.TestCase):

        if issubclass(TestClass, (unittest.TestCase, real_unittest.TestCase)):

            if len(parts) == 2: # label is app.TestClass

                try:

                    return unittest.TestLoader().loadTestsFromTestCase(TestClass)