`audio.stop()` & `microphone.stop_recording()` should only return when fully stopped

[microbit-carlos]

With audio it can take quite a few milliseconds before audio.is_playing() returns False:

>>> audio.play(microbit.Sound.HELLO, wait=False); audio.stop(); audio.is_playing()
True
>>> audio.play(microbit.Sound.HELLO, wait=False); audio.stop(); sleep(6); audio.is_playing()
False

With microphone the wait is a lot shorter, but still present:

>>> af = audio.AudioFrame(1000)
>>> microphone.record_into(af, wait=False); microphone.stop_recording(); microphone.is_recording()
True
>>> microphone.record_into(af, wait=False); microphone.stop_recording(); sleep(1); microphone.is_recording()
False