Adds TestFSMRef to TestKit

Fixes #312

`TestFSMRef` is used to unit test `FSM`s.
Instead of creating it with `ActorOf` use `ActorOfAsTestFSMRef`
This will give access to some of the internals of the FSM.


This builds upon PR #406. But I wanted to submit this as reviewing seems to be very slow right now, so instead of waiting for #406 to be accepted this can be reviewed now.

Only the last two commits are relevant (the rest are reviewed in #406)
https://github.com/HCanber/akka.net/commit/4fa2686ef34bc3d511150f88756f8836abf5a8d2 __FSM: Add nongeneric getters in Event__
https://github.com/HCanber/akka.net/commit/1ac43ba25515ff3b088b73e6b3347dee9fad4ff4 __	Implement TestFSMRef__

Nothing controversial in this PR.

FSM has some members that TestFSMRef needed access to. This was done by
having FSM implement InternalSupportsTestFSMRef (that contains those members)
explicitly.

__DO NOT merge this before #406__