added testcase for fields command
added testcase for fields command

first check of function without check of consistent output

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