Test kernel/kqueue/t_timer, subtests abstime, basic_timer and timer_units
often fail when run on QEMU because QEMU misses clock interrupts.

Always check timespec against expected "tv_sec" and use an

Test kernel/kqueue/t_timer, subtests abstime, basic_timer and timer_units
often fail when run on QEMU because QEMU misses clock interrupts.

Always check timespec against expected "tv_sec" and use an "4 * tv_sec"
upper bound when run under QEMU.

Now becomes part of PR kern/43997 "Kernel timer discrepancies".

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In the "modify" test case, immediately after modifying the timer, validate
that its associated knote in the kernel has actually been deactivated.

Support modifying an existing timer without having to delete it first.
Semantics match FreeBSD.

Add support for the NOTE_SECONDS, NOTE_MSECONDS, NOTE_USECONDS,
NOTE_NSECONDS, and NOTE_ABSTIME filter flags to EVFILT_TIMER,
API-compatible with the same in FreeBSD.