`BaseAdminIntegrationTestCase` should use `USERNAME_FIELD`

Username login field can be customised in Django. That means the `User` model wouldn't have necessarily a `username` field. Probably best to rely on `USERNAME_FIELD`.
Good question, it looks like the `username` depends on the authentication backend. As long as we use `django.contrib.auth.backends.ModelBackend` (Django default auth backend) we should be good. Otherwise the `setUp` should be customised to send what is appropriate to login someone.