kqueue tests: Add new EVFILT_TIMER regression tests from upstream

One of the tests exposes the regression reported in PR 264131.

One test is disabled because FreeBSD does not support setting EV_ONESHOT
on an already-added periodic timer. Though, in this case the flag is
simply ignored, which isn't ideal.

One test is slightly modified to set EV_ADD when reconfiguring a
disabled timer per some commentary in PR 258412.

Ideally we would re-import the test suite from libkqueue but there is a
fair bit of divergence so this will require some effort. This just gets
us one small step closer while increasing test coverage.

PR: 258412
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation