MFC 351481, 357619:

Approved by: bcr (mentor), hselasky (src)
Differential Revision: https://reviews.freebsd.org/D24058

mixer(8): Report an error if the passed value is an empty string

This patch fixes a bug that made the mixer command enter
an infinite loop when instructed to set the value of a device
to an empty string (e.g., mixer vol "").

Additionally, some tests for mixer(8) are being added.

PR: 240039
Reviewed by: hselasky, mav
Approved by: src (hselasky, mav)
Differential Revision: https://reviews.freebsd.org/D21409

mixer: call the cleanup function in a test

The set_empty_value test has a cleanup function, but is not called.
Fix it

Reviewed by: 0mp
Approved by: kp
Differential Revision: https://reviews.freebsd.org/D23498