remove skeleton tests for mobile_frontend and mobile_mail_frontend
(fixes #3464, BP from #3431)
(cherry picked from commit 5542d6174689fe6951093e37f493f8aaa5c892f4)
remove skeleton tests for pc_frontend (fixes #3464, BP from #3431)
(cherry picked from commit 52010fe24dd57de0c5265689f96986677452c5e7)
remove skeleton tests for mobile_frontend and mobile_mail_frontend
(fixes #3464, BP from #3431)
Merge branch 't-3464' into stable-3.8.x