mtree: Fix BSD.tests.dist sorting

Reviewed by: imp
Approved by: emaste (mentor)
Fixes: 60d717baf214 ("ctl: add some ATF tests for READ BUFFER")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47719

(cherry picked from commit 4b8e65d12fd7c06c0f670bdb11815971c3bebfed)