Fix test-runner on FreeBSD

CLOCK_MONOTONIC_RAW is only a thing on Linux and macOS. I'm not
actually sure why the previous hardcoding of a constant didn't
error out, but when we removed it, it sure does now.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12995