sound tests: Add SNDSTIOC_ADD_USER_DEVS test

Test whether the SNDSTIOC_ADD_USER_DEVS IOCTL (registers a userland
device to /dev/sndstat) works properly.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D46228