Fixed abstract-inclusive registered models listing with app specified.

The various get_models methods of Django don't give an option to include
abstract models, so a somewhat different solution to filter models by app
was needed. This had not been a problem for management commands as they
don't deal in abstract models, nevertheless should be fixed for other use
cases.
Made test_registration only count models from modeltranslation.tests app.

The only thing that was preventing the test from failing in the context
of a project with its own models registered for translation was the
translator registry duplication, which was removed in 1e566f4e.

The models count change is due to no longer considering the single
registered model from "managed_app".