Fixed #16574 -- Strengthened the `contrib.messages` tests' isolation to avoid...

Fixed #16574 -- Strengthened the `contrib.messages` tests' isolation to avoid failures under certain project setting environments. Thanks to Boldewyn for the report and to Claude Paroz for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17027 bcc190cf-cafb-0310-a4f2-bffc1f526a37