Mocking objects in tests

[dietrichm]

Currenty, the tests are quite heavy, since they initialize big parts of the system only to test one small component.

The code needs to be adapted so that object mocking is feasible. Afterwards, the existing tests need to be changed.

[dietrichm]

After a long period of silence, I continued this and now half of the tests are reworked with mocks and 100% coverage. This ticket should be done within a week (or two).