info: add unit tests
info: add unit tests

* Add tests for length (human/raw, library/path) and custom format.

madmouser1/beets,Kraymer/beets,xsteadfastx/beets,sampsyo/beets,mosesfistos1/beetbox,lengtche/beets,swt30/beets,beetbox/beets,SusannaMaria/beets,mried/beets,MyTunesFreeMusic/privacy-policy,pkess/beets,kelvinhammond/beets,xsteadfastx/beets,mried/beets,pkess/beets,madmouser1/beets,kelvinhammond/beets,xsteadfastx/beets,lengtche/beets,shamangeorge/beets,diego-plan9/beets,Freso/beets,LordSputnik/beets,shamangeorge/beets,sampsyo/beets,shamangeorge/beets,jcoady9/beets,Kraymer/beets,artemutin/beets,LordSputnik/beets,swt30/beets,diego-plan9/beets,diego-plan9/beets,beetbox/beets,LordSputnik/beets,ibmibmibm/beets,mosesfistos1/beetbox,kareemallen/beets,parapente/beets,kareemallen/beets,Freso/beets,Freso/beets,beetbox/beets,LordSputnik/beets,Freso/beets,jackwilsdon/beets,parapente/beets,sampsyo/beets,diego-plan9/beets,lengtche/beets,jackwilsdon/beets,SusannaMaria/beets,kareemallen/beets,Kraymer/beets,SusannaMaria/beets,Kraymer/beets,SusannaMaria/beets,kelvinhammond/beets,jcoady9/beets,MyTunesFreeMusic/privacy-policy,pkess/beets,jackwilsdon/beets,ibmibmibm/beets,madmouser1/beets,MyTunesFreeMusic/privacy-policy,sampsyo/beets,parapente/beets,madmouser1/beets,xsteadfastx/beets,parapente/beets,kareemallen/beets,jackwilsdon/beets,kelvinhammond/beets,jcoady9/beets,pkess/beets,lengtche/beets,ibmibmibm/beets,beetbox/beets,mosesfistos1/beetbox,shamangeorge/beets,MyTunesFreeMusic/privacy-policy,mried/beets,artemutin/beets,artemutin/beets,mried/beets,swt30/beets,jcoady9/beets,ibmibmibm/beets,artemutin/beets,mosesfistos1/beetbox,swt30/beets