sound tests: Test polling

Co-authored by: meka@tilda.center
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53188

(cherry picked from commit dd81b19ef23622d03d7c75935e11c2b2af3f4cb8)
(cherry picked from commit 41f2ec3be93a2981e5afcc2569e6abcc64414437)
(cherry picked from commit eb95b990f8ebfb85ce6e4d7d16305746b5eddcde)
(cherry picked from commit fb99c0ba60dc464aab2102c4395791e151d438c6)