mtree: Add missing entries in BSD.tests.dist

Reviewed by: emaste, christos
Approved by: emaste (mentor)
Fixes: 95eb4b873b6a ("sound tests: Add sndstat nvlist ATF test")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47724

(cherry picked from commit f08dfd819bf73a584bf90da268242a7ed483dc32)