This fixes a few race conditions in `test_waker_threaded()` and adds a
timeout to ensure that it doesn’t hang forever if something goes wrong.

Some notes:

  * It is always possible to have messages on the channel after the
    thread has finished, since the thread could write a message and
    finish between the call to `poll()` and the start of the next loop.
  * `reset()` could swallow the last wake.
  * It is possible to get either timeouts or events from `poll()` after
    the thread finishes. I’m not sure how it gets more events, but when
    I only checked for `is_finished()` when getting a timeout, it
    sometimes hung forever.

Fixes: cloudhead#19 — test_waker_threaded fails or hangs in Rust 1.70