Fixed #20916 - Added support for a `simple_login` test client method.

`simple_login` takes a user instance and logs you in as that user. You no longer have to manage user credentials in tests.

Ticket: 20916.

Todo:
* [ ] Add docs
This change isn't correct as it will disable the check unless testing. Maybe this would work: `if 'django.contrib.auth.backends.ModelBackend' in settings.AUTHENTICATION_BACKENDS`