Fixed the auth tests so they work when the AUTHENTICATION_BACKENDS setting is...

Fixed the auth tests so they work when the AUTHENTICATION_BACKENDS setting is a list.  Thanks to Patrick Altman for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14406 bcc190cf-cafb-0310-a4f2-bffc1f526a37