The test was expecting connections used by DiscoverRunner.setup_databases()
to be the ones defined in django.test.runner but this doesn't hold true
since this method was made a proxy of django.test.utils.setup_databases.

This broke the TransactionTestCase.serialized_rollback feature in the test
suite because calls to create_db_test() cleared the test data persisted on
connections objects.

Added an assertions to prevent this from happening again.