slight improvement on `fudge` detection
Call `super()` on ArmstrongTestCase. A good idea, but not strictly necessary as DjangoTestCase doesn't have its own `setUp()` method.
Fix and simplify `generate_random_users()`. Randomization wasn't guaranteed and I've seen it break with `IntegrityError: column username is not unique`.

The new function is more generic and it's a generator. Basically the same fix as armstrong/armstrong.core.arm_layout#14.
This is only used in ArmContent, ArmAccess and AppsDonations. The most extensive use is in ArmContent, which is being updated concurrently. Swapping in the new method will be simple.
Remove Django magic that makes concrete models. Only used in ArmContent testing and that is being refactored concurrently.