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.
Merge pull request #3331 from carlosviol/develop

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