Moved an import in an auth test; refs #23925.

This keeps tests/__init__.py from importing other modules and may fix a problem
with test discovery revealed in formtools tests on Travis CI.

django/contrib/auth/tests/__init__.py

 # The password for the fixture data users is 'password'
-
-# For testing that auth backends can be referenced using a convenience import
-from django.contrib.auth.tests.test_auth_backends import ImportedModelBackend  # NOQA

django/contrib/auth/tests/backend_alias.py

+# For testing that auth backends can be referenced using a convenience import
+from django.contrib.auth.tests.test_auth_backends import ImportedModelBackend  # NOQA

django/contrib/auth/tests/test_auth_backends.py

@@ -620,7 +620,7 @@ class ImportedBackendTests(TestCase):
     as the one defined in AUTHENTICATION_BACKENDS setting.
     """
 
-    backend = 'django.contrib.auth.tests.ImportedModelBackend'
+    backend = 'django.contrib.auth.tests.backend_alias.ImportedModelBackend'
 
     @override_settings(AUTHENTICATION_BACKENDS=(backend, ))
     def test_backend_path(self):