[1.8.x] Isolated a flatpages test; refs #11505.

Backport of 4135d83702 from master

django/contrib/flatpages/tests/test_sitemaps.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 from django.apps import apps
+from django.contrib.sites.models import Site
 from django.test import TestCase
 from django.test.utils import modify_settings, override_settings
 
@@ -12,6 +13,13 @@ from django.test.utils import modify_settings, override_settings
 @modify_settings(INSTALLED_APPS={'append': ['django.contrib.sitemaps']},)
 class FlatpagesSitemapTests(TestCase):
 
+    @classmethod
+    def setUpClass(cls):
+        super(FlatpagesSitemapTests, cls).setUpClass()
+        # This cleanup is necessary because contrib.sites cache
+        # makes tests interfere with each other, see #11505
+        Site.objects.clear_cache()
+
     @classmethod
     def setUpTestData(cls):
         Site = apps.get_model('sites.Site')