tests: kqueue: use a more precise timer for the NOTE_ABSTIME test

Originally noticed while attempting to run the kqueue tests under
qemu-user-static, this apparently just happens sometimes when running in a
jail in general -- the timer will fire off "too early," but it's really just
the result of imprecise measurements (noted by cem).

Kicking this over to NOTE_USECONDS still tests the correct thing while
allowing it to work more consistently; a basic sanity test reveals that we
often end up coming in just less than 200 microseconds after the timer
fired off.

MFC after: 3 days