Support @Spy AbstractClass/Interface

Had to fix a few existing tests because ```spy(Class)``` doesn't like private ocnstructors.

I think the most common case would be for a test to ```@Spy PrivateNestedClass```. By default, private class's default constructor is private so these tests were implicitly calling the private constructor.

Another option would be to check and skip calling constructor for private constructors. Although, that would be behavior-changing too.
(not part of this RB) might be worth detecting if we're mocking inner class but the enclosing instance does not match. We could throw more meaningful exception this way.