additional platform agent tests to check robustness

@edwardhunter please review/merge

Noteworthy:
- re-enabled test of launching network of 13 platforms and 8 instruments (in test_platform_launch.py)
- new test_platform_agent_robustness.py with tests exercising adverse sequences (instrument reset externally; activation/deactivation and reception of device_failed_command events; combination of agents put into streaming; instrument terminated externally and graceful handling by platform)
- more strict verification of expected mission events (not only verifying the expected number but also the contents of such events)

@bobfrat FYI