Fixed backref name of the groups_user join table

* Fixed package path in testsetup. The testrunner wronlgy searched the complete project, so also the `src` directory for tests, which end in import errors. With this change the runner only runs test from the opengever.core package.

* Fixed backref name of the groups_user join table. See 4teamwork/opengever.ogds.models#4 for more details.