test_subscriber waits blindly for ignition gazebo to start

[FirefoxMetzger]

Testing the ignition interface opens the simulator to interface with it. Currently, this is done via subprocess.Popen and without any checks that the simulator is ready. Instead, we just time.sleep(3) and hope it will have initialized after this time. This works most of the time, but might randomly break. It also wastes time as the test may wait much longer than it needs to.

What is needed is a mechanism to detect if ignition is running and has finished initializing.