Fixed #16925 -- Make sure a signal is disconnected if the test fails. Thanks...

Fixed #16925 -- Make sure a signal is disconnected if the test fails. Thanks to aaugustin for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16905 bcc190cf-cafb-0310-a4f2-bffc1f526a37