Fixed multiple database tests that broke foreign keys constraints. Refs #17055.

The problem was masked by the rollback at the end of each transactional test on backends that deferred constraints checks; it appeared only on MySQL + InnoDB.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@16995 bcc190cf-cafb-0310-a4f2-bffc1f526a37