Our TestModelsLoaderMixin was never 100% updated when Django added

AppConfig, and this meant that the tear-down for test cases that define

a tests_app wasn't quite correct. The app got properly removed from

INSTALLED_APPS, but we were no longer removing models correctly.

This manifested in test cases that use the djblets.integrations.tests

app.
The fix here involves a couple parts: the mixin has been updated to

properly look up the app config and use its label, and a new AppConfig

has been added for djblets.integrations.tests that defines a label so

it doesn't just get turned into "tests".

Ran unit tests and no longer saw ugly warnings about models already

being registered.