One pattern I like to use for stuff like this is follow QuickCheck and not specify the fields that are uninteresting, but instead generate them randomly. I haven't put together a library but here is what I mean:

Tests: https://github.com/dimagi/django-prbac/blob/master/django_prbac/tests/test_models.py

Generators: https://github.com/dimagi/django-prbac/blob/master/django_prbac/arbitrary.py