Improve TestCase API

[CohenArthur]

TestCase::new() and TestCase::Skip do not make a lot of sense when creating the two possible variants of a Test case. Furthermore, does it make sense to keep a builder pattern for non-skipped test cases? If the answer is yes, does it make sense to simply return Skip cases even when calling builder functions, effectively making the call a no-op? Should we panic!() to indicate that the user might be doing something unexpected or silently ignore that as we currently do?