test: fix test-net-settimeout flakiness

Wait for the data to be received by the socket before creating the
clean-up timer. This way, a possible (though unlikely) `ECONNRESET`
error can be avoided.

PR-URL: https://github.com/nodejs/node/pull/6166


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>