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

usr.sbin/mixer/tests/mixer_test.sh

@@ -66,7 +66,8 @@ set_default()
 
 restore_default()
 {
-	test -r "test_mixer_deflt_unit" && mixer -d $(cat test_mixer_deflt_unit)
+	test -r "test_mixer_deflt_unit" &&
+	mixer -d $(cat test_mixer_deflt_unit) || true
 }
 
 atf_test_case o_flag cleanup