Fixed tests introduced for #15915

The tests didn't clean up properly. The commit that introduced the
errors was 8c42744.

Thanks to Trac alias rizumu for spotting this.

django/contrib/auth/tests/management.py

@@ -173,10 +173,10 @@ def test_swappable_user_missing_required_field(self):
 class PermissionDuplicationTestCase(TestCase):
 
     def setUp(self):
-        self._original_user_permission = models.User._meta.permissions
+        self._original_permissions = models.Permission._meta.permissions[:]
 
-    def tearUp(self):
-        models.User._meta.permissions = self._original_user_permissions
+    def tearDown(self):
+        models.Permission._meta.permissions = self._original_permissions
 
     def test_duplicated_permissions(self):
         """