Fix problem with testsuite modifying migrated model auth.Group

Django 1.7 comes with its own migrations framework and has bundled
migrations for all models. The test which tried to run `mptt.register()`
on auth.Group always failed because Django really does not want database
changes without migrations anymore.

We can easily substitute auth.Group with our own Group model: the test
suite only checks whether saving groups works (and saving would fail if
the database would not contain MPTT's internal fields for some reason.)