MPD keeps the current track in the queue when stopping, so it's not
really like a skip, and I use it so that I can stop the music, and later
start at the beginning of a track.
I do this by keeping track of the current song id, and then comparing
them when we receive a stop signal.
This was a vestige from when we used to need the unittest2 library for pre-2.7
compatibility. Now that we require Python 2.7, we aren't using that library
and this indirection wasn't doing any good.