Currently InputMethodMus instance is created directly in WindowTreeHostMus, therefore it doesn't support mocking in the tests.

Ideally, WindowTreeHostMus should leverage InputMethodFactory to create the input method instance. The test/mock setup is handled there.
However, the InputMethodMus constructor requires different parameters, and some test code requires to access the internal members of InputMethodMus (e.g. InputMethodMusTestApi).

So some refactoring needs to be done before InputMethodMus can be mock'ed.