Small change in tutorial of how to test extensions and their urls, useful for nosetests users

Small change in tutorial of how to test extensions and their urls. Needed because we had a conflict with nosetests (django-nose), that was loading all test_* by default, and we thought that maybe be a common conflict of a lot of users that also use nosetests with django-cms and follow this doc.
    
This problem was that url resolver called the database before of the test database configuration when nosetests loaded test_urls.py, so the django_sites and all tables were not created yet.