BPD tests: improve test helpers
BPD tests: improve test helpers

Decode the bytes to strings: the MPD protocol specifies that the
communications are all in UTF-8.

Also parse the body into a dict since this is typically more convenient
than having to do it manually in each test.

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