[1.2.X] Fixed the auth tests so they work when the AUTHENTICATION_BACKENDS...

[1.2.X] Fixed the auth tests so they work when the AUTHENTICATION_BACKENDS setting is a list.  Thanks to Patrick Altman for the report.  Backport of [14406].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14407 bcc190cf-cafb-0310-a4f2-bffc1f526a37