convert more tests to nmock3

- Scene and SceneFactory now have interfaces
- converted TestSpriteCollection and TestSceneManager
make classes mockable for TestMessagePrompt

- made several methods virtual
- extracted the AbstractMessageBox interface
- Changed Prompt so that it doesn't create GameObjects in its
  constructor
made MockUnity work better with NMock3

made several methods virtual and cleaned up some mocks.
converted two more test classes to NMock3

Only one to go!