Hang dump should offer activity start and stop

[Youssef1313]

Instead of relying on hardcoded test node states, hang dump should introduce new properties for the start and stop of an activity. This would match better the open states, states workflow model.

Besides, it would enable to extend the principle to non nodes objects. For example, on TATF, we could use the activity principle to signal if some fixtures are getting stuck, which could help the user debugging the hang (e.g. all tests are done but I see some fixture started but not closed, I might want to inspect the fixtures first).

Originally posted by @Evangelink in https://github.com/microsoft/testanywhere/issues/1212

[Evangelink]

Same comment about this probably being only a far future need.