[1.8.x] Isolated auth_tests from contenttypes_tests; refs #11505.

Backport of e0b3926026 from master

tests/auth_tests/test_forms.py

@@ -9,6 +9,7 @@ from django.contrib.auth.forms import (
     UserChangeForm, UserCreationForm,
 )
 from django.contrib.auth.models import User
+from django.contrib.sites.models import Site
 from django.core import mail
 from django.core.mail import EmailMultiAlternatives
 from django.forms.fields import CharField, Field
@@ -362,6 +363,13 @@ class PasswordResetFormTest(TestCase):
 
     fixtures = ['authtestdata.json']
 
+    @classmethod
+    def setUpClass(cls):
+        super(PasswordResetFormTest, cls).setUpClass()
+        # This cleanup is necessary because contrib.sites cache
+        # makes tests interfere with each other, see #11505
+        Site.objects.clear_cache()
+
     def create_dummy_user(self):
         """
         Create a user and return a tuple (user_object, username, email).