discogs: add more unit tests
discogs: add more unit tests

Add unit tests covering the discogs standard positions (12.2.9). Clean
up a bit the helper methods (unused parameters, documentation).

https://www.discogs.com/help/submission-guidelines-release-trk.html#Position

lengtche/beets,sampsyo/beets,xsteadfastx/beets,SusannaMaria/beets,lengtche/beets,xsteadfastx/beets,MyTunesFreeMusic/privacy-policy,diego-plan9/beets,SusannaMaria/beets,madmouser1/beets,madmouser1/beets,beetbox/beets,pkess/beets,pkess/beets,ibmibmibm/beets,lengtche/beets,jackwilsdon/beets,xsteadfastx/beets,MyTunesFreeMusic/privacy-policy,mosesfistos1/beetbox,Kraymer/beets,madmouser1/beets,diego-plan9/beets,ibmibmibm/beets,artemutin/beets,pkess/beets,diego-plan9/beets,sampsyo/beets,artemutin/beets,SusannaMaria/beets,Kraymer/beets,madmouser1/beets,mosesfistos1/beetbox,Kraymer/beets,ibmibmibm/beets,artemutin/beets,shamangeorge/beets,shamangeorge/beets,pkess/beets,jackwilsdon/beets,xsteadfastx/beets,jackwilsdon/beets,artemutin/beets,SusannaMaria/beets,jackwilsdon/beets,beetbox/beets,mosesfistos1/beetbox,ibmibmibm/beets,MyTunesFreeMusic/privacy-policy,beetbox/beets,sampsyo/beets,lengtche/beets,diego-plan9/beets,beetbox/beets,shamangeorge/beets,mosesfistos1/beetbox,shamangeorge/beets,sampsyo/beets,MyTunesFreeMusic/privacy-policy,Kraymer/beets