ZTS: Use inbuilt monotonic time

Make the test runner try to use the included python monotonic time
function instead of calling librt.

This makes the test runner work on macos where librt wasn't available.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Andrew Innes <andrew.c12@gmail.com>
Closes #14700