mixer(8) tests: Fix cleanup routine

The tests correctly skip if no snd_dummy neither mixer is found, but the
cleanup is still called with the skip condition, which fails if there is
no mixer.

MFC after: 2 days
Reviewed by: christos
Differential Revision: https://reviews.freebsd.org/D46491

(cherry picked from commit 080c85127e3fba2c8cfb78cb75f7b306aee4028d)