[1.2.X] Fixed #10420 -- GeoDjango tests are run as part of Django tests when...

[1.2.X] Fixed #10420 -- GeoDjango tests are run as part of Django tests when using spatial database backends with `runtests.py`.

Backport of r15013 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15014 bcc190cf-cafb-0310-a4f2-bffc1f526a37