Create helper fixtures for authenticated states on RequestFactory and client

[tony]

I think the below are widely enough used on Django projects tests to be part of pytest-django itself.

• Fixture to create a django user (similar to admin_user, but non-admin)
• Create a user_client (similar to admin_user, but non-admin, related Document use of authenticated client and django_user_model #553 Document use of authenticated client and django_user_model #554)
• Create a request factory for unauth users (for AttributeError: 'WSGIRequest' object has no attribute 'user' errors). Helps mimic user middleware fast of falling back on AnonymousUser. See also, this example in Django docs
• Request factory for normal django user
• Request factory for admin user Request factory with a preset admin user #284