timerfd: Add tests

Take Jan Kokemuller's timerfd tests from the epoll-shim project,
stripping out code that isn't directly related to FreeBSD.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55789
MFC after: 2 weeks

(cherry picked from commit 834c1ba793d900b791e19ff449b1e0b96fb494a0)