edit: update extra fields in yaml test
edit: update extra fields in yaml test

* Update test for extra fields in edited yaml, allowing the user to add fields
while editing. Rename the test to test_single_edit_add_field to reflect its
purpose more accurately.

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