bpd tests: use mp.Queue to communicate assigned port
bpd tests: use mp.Queue to communicate assigned port

Use a `multiprocessing.Queue` instead of a `multiprocessing.Value` to avoid the
manual polling/timeout handling.

TODO: Strangely Listener seems to be constructed twice. Only the second one is
used. Fix that and then remove the code working around it.

sampsyo/beets,jackwilsdon/beets,SusannaMaria/beets,beetbox/beets,shamangeorge/beets,ibmibmibm/beets,jackwilsdon/beets,shamangeorge/beets,jackwilsdon/beets,sampsyo/beets,SusannaMaria/beets,beetbox/beets,SusannaMaria/beets,SusannaMaria/beets,shamangeorge/beets,sampsyo/beets,shamangeorge/beets,ibmibmibm/beets,beetbox/beets,jackwilsdon/beets,ibmibmibm/beets,sampsyo/beets,beetbox/beets,ibmibmibm/beets