Add unit test integration to design-time-host

This allows VS to initiate test discovery or execution for a project.
That's another thing that would be sort of defined per-command yeah? What we could do here is just return the exit code to the caller and let them deal with it.

In terms of a test framework, I'm not terribly sure that it's meaningful since we already provide richer reporting. Let me know what you want me to do here.