ctl: add some ATF tests for READ BUFFER
Reviewed by: Pierre Pronchery <pierre@freebsdfoundation.org>
Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: Axcient
(cherry picked from commit 60d717baf2144cf344ec9b47d715ce837b5d46d4)
(cherry picked from commit e9329a869c8137b955b2b3487ac0dadabf540bda)