[1.5.x] Updated User manager when testing custom AUTH_USER_MODEL

This is giving more real test conditions when AUTH_USER_MODEL is
set with override_settings.
Backport of a962bc7c4 from master.

django/contrib/auth/tests/__init__.py

@@ -14,3 +14,16 @@ from django.contrib.auth.tests.tokens import *
 from django.contrib.auth.tests.views import *
 
 # The password for the fixture data users is 'password'
+
+from django.dispatch import receiver
+from django.test.signals import setting_changed
+
+
+@receiver(setting_changed)
+def user_model_swapped(**kwargs):
+    if kwargs['setting'] == 'AUTH_USER_MODEL':
+        from django.db.models.manager import ensure_default_manager
+        from django.contrib.auth.models import User
+        # Reset User manager
+        setattr(User, 'objects', User._default_manager)
+        ensure_default_manager(User)

django/contrib/auth/tests/basic.py

@@ -162,6 +162,8 @@ class BasicTestCase(TestCase):
     def test_swappable_user(self):
         "The current user model can be swapped out for another"
         self.assertEqual(get_user_model(), CustomUser)
+        with self.assertRaises(AttributeError):
+            User.objects.all()
 
     @override_settings(AUTH_USER_MODEL='badsetting')
     def test_swappable_user_bad_setting(self):