Improvements to the `ObjectTest` system

[Kroppeb]

• tests should be able to mark themselves as "supposed to fail", with an expected error type / message
• ideally we don't need to write a new class for each test case, instead, we could use an entry point that just works on a single field
• the builds are now failing due to tests for not yet implemented features, ideally we can mark them as such and make them be ignored by default.