TestAsyncUDPSocket: remove dead code and add test

- _ThreadedClient was added on the idea of making this unittest
  concurrent, which was actually never done (we could test everything
  without it, so well)
- handle_write() was never called without filling the send queue, and
  this caused me trouble now that I learned to look at coverage

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>