[1.2.X] Converted tests for contrib.auth.forms to unit tests.

Backport of [13701], needed in order to backport [13702]


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13703 bcc190cf-cafb-0310-a4f2-bffc1f526a37