In completion tests, don't load system scripts
In completion tests, don't load system scripts

On my machine, the "leak" of other bash_completion scripts here recently
started doing a *lot* of work for other packages. Most damningly, it started
compiling a bunch of stuff with rustc! Now we only load beets' own completion
stuff.

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