OGM-45 - Unifying and cleaning up test set up

* Retrofitting `OgmTestCase` to support JUnit 4 functionality
* Removing unused functionality
* Moving `OgmTestCase` to `utils` package

~~Creating this preview PR as base for discussion. The idea is to provide a JUnit 4 runner for OGM tests which does the session factory handling, without requiring tests to extend a superclass.~~

~~Alternatively a JUnit 4 _test rule_ could be used. But this has several shortcomings:~~

~~* It can't access methods of the test class, so something like `configure()` above isn't possible. Given that we currently only use this method to set properties, one could think about an annotation based approach for setting these. One can't invoke something like `AssociationStorage.COLLECTION.name()` within an annotation, though.~~
~~* AFAICS a rule can't suppress single test methods, which we would need for `@SkipDialect` (which is apparently not used though atm.).~~

~~Based on that I think a runner is more suitable here. Any thoughts?~~