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 https://github.com/4teamwork/opengever.ogds.models/pull/4 for more details.
Merge pull request #233 from 4teamwork/pg_fix_backref_name

 Fixed backref name of the groups_user join table