Made some `contrib.messages` tests use `override_settings` for clarity. Thanks...

Made some `contrib.messages` tests use `override_settings` for clarity. Thanks to Claude Paroz for the patch. Refs #16507, #16574.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17032 bcc190cf-cafb-0310-a4f2-bffc1f526a37