mixer/tests: use require.kmods property instead of ad-hoc checks

Reviewed by:	christos
Approved by:	lwhsu (mentor)
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D55221

usr.sbin/mixer/tests/Makefile

@@ -1,6 +1,7 @@
 ATF_TESTS_SH+=	mixer_test
 
 # mixer tests fiddle with a singleton dummy audio device.
-TEST_METADATA.mixer_test=	is_exclusive=true
+TEST_METADATA.mixer_test=	is_exclusive=true \
+				required_kmods="snd_dummy"
 
 .include <bsd.test.mk>

usr.sbin/mixer/tests/mixer_test.sh

@@ -45,13 +45,6 @@ restore_conf()
 	test -r "test_mixer_conf" && mixer $(cat test_mixer_conf)
 }
 
-load_dummy()
-{
-	if ! kldload -n snd_dummy; then
-		atf_skip "cannot load snd_dummy.ko"
-	fi
-}
-
 set_default()
 {
 	deflt_unit="$(mixer | grep ^pcm | cut -f1 -d:)"
@@ -78,7 +71,6 @@ o_flag_head()
 }
 o_flag_body()
 {
-	load_dummy
 	mixer_exists
 	set_default
 
@@ -96,7 +88,6 @@ d_flag_head()
 }
 d_flag_body()
 {
-	load_dummy
 	mixer_exists
 	set_default
 
@@ -118,7 +109,6 @@ volume_head()
 }
 volume_body()
 {
-	load_dummy
 	mixer_exists
 	set_default
 	save_conf
@@ -204,7 +194,6 @@ mute_head()
 }
 mute_body()
 {
-	load_dummy
 	mixer_exists
 	set_default
 	save_conf
@@ -248,7 +237,6 @@ recsrc_head()
 }
 recsrc_body()
 {
-	load_dummy
 	mixer_exists
 	set_default
 	save_conf