Convert the contettypes tests to use self.assertNumQueries, rather than hand...

Convert the contettypes tests to use self.assertNumQueries, rather than hand rolling their own version. Thanks to carl for the review.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16962 bcc190cf-cafb-0310-a4f2-bffc1f526a37