the previously used std::this_thread::sleep_for is with respect to host
system time which is different from autopilot time if:
- speed factor != 1
- something runs slower than realtime regardless of speed factor
- debugging or otherwise interrupting PX4 control code
tester.sleep_for (which already existed) correctly sleeps w.r.t.
px4/simulation time.
wait_until_altitude() checks for absolute altitude being
close so checking for 1m below the setpoint can fail
if the speedup results in no sample inside the altitude
window being checked.
Ideally the test could check if the takeoff is done directly
instead of comparing altitudes in the first place.
