Improve Mock support for Viewcontexts

[rynowak]

Discovered during filters work. All of our old context classes in MVC had a no-arg constructor + virtual public property accessors for testability. Need to do a pass

[Eilon]

BTW this is often for app developer unit tests; though we of course also benefit from this for our own unit tests.

[ianhong]

ActionContext will be updated per #1934