Make Django 1.7 'check' warning go away by specifying a test runner

I don't really understand the details tbh - I just followed the advice
here:
http://daniel.hepper.net/blog/2014/04/fixing-1_6-w001-when-upgrading-from-django-1-5-to-1-7/
Ensure US site works with Django 1.7

Needed to fiddle with South and copy the new migrations from the
corresponding Oscar apps.

Spotted this while looking into #1479.