sound: Unit test the pcm sample read and write macros

Main goal is to have a unit test, with sample test data that is verified
against the current macro implementation of pcm sample read and write
functions. With a test in place, we can proceed on a planned refactoring
of the sample read and write code, and confidently check the new code
for regressions.

Implementation of the unit test itself has to avoid any cast or
conversion affected by endianness, to make the tests compatible with all
machine architectures.

MFC after: 1 week
Reviewed by: christos, markj
Differential Revision: https://reviews.freebsd.org/D48330