Fix login-required test when run without Mezzanine accounts

The test only fails when run without ``mezzanine.accounts`` in ``INSTALLED_APPS`` -- for example when testing a newly created project using ``manage.py test``.

The failure is due to the redirect target not existing, ``/accounts/login`` is only defined by the accounts module; ``contrib.auth`` doesn't seem to provide any default login page (other than possibly the ``contrib.admin`` one).