tests/kqueue: repair flaky test

[glebius]

Summary

The test arms timer for 500 msec and then sleeps 1 second and expects that
the timer would always fire exactly 2 times. However, it has a chance to
legitimately fire only once. This makes the test flaky. Fix the test
reducing sleep time down to 450 msec.

The problem started to show up recently, and I attribute that to
626ea75ed2e9. Before this revision sleep(3) was almost always sleeping
over 1000 msec, now it can sleep exactly 1000 msec.

[markj]

tests/sys/kqueue/libkqueue/timer.c

202

Thinking a bit more, I would probably try to avoid such a large error. Do you still see occasional test failures if the timeout is, say, 495ms?

[glebius]

Agreed, let's try 495 and see what CI reports after a week.