Tweaked a tests tweak from r16960 so it uses the right model.

When testing for permission enforcement in the admin for m2m
relationships we were erroneously using the ID of the related model
instead of using the ID of the corresponding automatically created
intermediate model. This manifested as a failure under Oracle but can
fail with any backend if the IDs of both models diverge.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17001 bcc190cf-cafb-0310-a4f2-bffc1f526a37