Sometimes, simctl has stderr logging, which should be generally ignored when listing devices since that logging will not be json.
<rdar://problem/63517635>
Turns out that this applies to other simctl commands too.
Created attachment 400246 [details] Patch
Committed r262143: <https://trac.webkit.org/changeset/262143> All reviewed patches have been landed. Closing bug and clearing flags on attachment 400246 [details].