make xctool work with Kiwi & Specta again

We broke support for Kiwi and Specta about a month ago :cry: 

This fixes the test case introspection stuff to be compatible with the magic that Kiwi & Specta both do with `+[SenTestCase testInvocations]`.

This also changes the way we invoke _otest_ or _xctest_ when running specific test cases - instead of passing the exact list of tests to be run, we pass the list of tests we do _not_ want to run and set `-SenTestInvertScope YES`.

Tested manually with projects that contained Kiwi & Speca tests.

Closes #202