tests: kqueue: CLOCK_BOOTTIME is an alias of CLOCK_UPTIME

Build-test should be done a buildenv from a newer branch. =-( We don't
have this alias in stable/12, so just provide it locally in a way that
won't break should 155f15118a77 find its way here.

Reported by: Jenkins, naddy
Fixes: f9765c9c1acb ("kqueue: don't arbitrarily restrict ...")