t_resourceassigner.py

SW-816: bonus feature: refactored RADBCommonTest into a RADBCommonTestMixin which takes care of setting up a testing postgres radb instance, and which can be used in conjunction with a unittest.TestCase to do unittests on a fresh RADB instance. By using a mixin instead of chained inheritance we have better seperation of responsibility, and tools like pycharm can handle these tests better as well.