Fixed #15111 -- Ensured that the auth, contenttypes and sitemaps tests will run when the sites app isn't installed. Thanks to Waldemar Kornewald for the report and draft patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15418 bcc190cf-cafb-0310-a4f2-bffc1f526a37