Fixed #15753 -- Cleared cache between sitemaps tests

When caching was activated, test_simple_sitemap would fail
because the test result was fetched from cache.
Thanks lucho for the initial patch and krzysiumed@gmail.com for
the review.

django/contrib/sitemaps/tests/base.py

 from django.contrib.auth.models import User
 from django.contrib.sites.models import Site
+from django.core.cache import cache
 from django.test import TestCase
 
 
@@ -11,6 +12,7 @@ class SitemapTestsBase(TestCase):
     def setUp(self):
         self.base_url = '%s://%s' % (self.protocol, self.domain)
         self.old_Site_meta_installed = Site._meta.installed
+        cache.clear()
         # Create a user that will double as sitemap content
         User.objects.create_user('testuser', 'test@example.com', 's3krit')