[1.1.X] Modified the delete test to clean up after itself properly, so that...

[1.1.X] Modified the delete test to clean up after itself properly, so that the proxy_model_inheritance test can run without requiring manual confirmation.

Backport of r12850 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12851 bcc190cf-cafb-0310-a4f2-bffc1f526a37