change verification logic for test_simple_mission_streaming_state: now only based on expected events from mission executive
change verification logic in most tests in test_mission_manager.py to be based on expected events from mission executive
minor log change to avoid unnecessary warning
update verification in test_ao_turn_off_platform_port
make verification in _test_multiple_missions also based on expected events, in this case state transition events (ResourceAgentStateEvent)
comment out log message
remove unneeded method external_event_received in MissionManager
convenient additional parameter to _start_event_subscriber2 to facilitate logging for particular callers
all tests now also verifying expected state transition events
remove unneeded file
fix check for current state when first mission is to be executed
some additional logging in test
Merge pull request #2175 from carueda/pa_mx_mission_in_streaming

M101: much more robust verification of mission execution tests