gh-110697: test_os TimerfdTests uses selectors (#110789)

Replace select.epoll() with selectors.DefaultSelector to support
FreeBSD 14 and newer.

* Merge common code between test_timerfd_epoll()
  test_timerfd_ns_epoll().
* Replace "_ = func()" with "func()".
* Add TimerfdTests.read_count_signaled() method.