M101: much more robust verification of mission execution tests

@edwardhunter please review/merge

```
bin/nosetests -sv  --with-pycc ion/agents/platform/test/test_mission_manager.py
...
Ran 7 tests in 1307.305s

OK
```

@bobfrat FYI --  BTW, as in test_simple_mission_streaming_state, we could also indicate the specific expected events in the other tests for even further validation.