None of the existing bok-choy tests in edx-platform reference the Django settings, because the tests are running against servers in separate processes with two different sets of settings (one for LMS, one for Studio). You should be able to work around the need for AUTH_USER_MODEL by using django.contrib.auth.models.User directly, but you generally shouldn't try to work with Django models in bok-choy tests. You can import some code from the openedx and xmodule packages in the bok-choy tests, but without Django running in the same process it usually isn't very useful. The vast majority of our test cases rely primarily on the selenium and bok-choy APIs to work with the browser, not the server implementation.