mavsdk_tests: better process management
mavsdk_tests: better process management

We now error if px4 or gzserver are already running on startup, and we
also make sure to terminate what we started if interrupted.

acfloria/Firmware,PX4/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware