Remove references to AppUser to allow using custom models structure

[superbiche]

Hey there :)

We're using a custom-but-quite-common models structure, using App\Models instead of App as base namespace.
Thus, ModerationScope using App\User is raising errors in our case, and so is the BaseTestCase acting as App\User.

Therefore, I only removed the use statement in ModerationScope (wasn't used), and used adamwathan/eloquent-oauth method of retrieving the user class from config('auth.providers.users.model', config('auth.model', 'App\User')) for BaseTestCase.

Hope it's okay, and thanks for this great package.
Cheers,

[hootlex]

Hey,

Thanks for the PR and the detailed description.
My first thought was to document the auth.model config option but since it's only used in the tests I think it will just be more confusing. So, merging right away! :)