[1.8.x] Fixed #24345 -- Isolated sitemaps_tests from contenttypes_tests; refs #11505

Backport of f668bac9 from master

tests/sitemaps_tests/base.py

 from django.apps import apps
+from django.contrib.sites.models import Site
 from django.core.cache import cache
 from django.test import TestCase, modify_settings, override_settings
 
@@ -18,3 +19,10 @@ class SitemapTestsBase(TestCase):
         # Create an object for sitemap content.
         TestModel.objects.create(name='Test Object')
         self.i18n_model = I18nTestModel.objects.create(name='Test Object')
+
+    @classmethod
+    def setUpClass(cls):
+        super(SitemapTestsBase, cls).setUpClass()
+        # This cleanup is necessary because contrib.sites cache
+        # makes tests interfere with each other, see #11505
+        Site.objects.clear_cache()