replaygain: add target_level test
replaygain: add target_level test

Assert that analysing the same track with different target levels yields
different gain adjustments.

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