test: fix flaky test-http-set-timeout-server

Make the servers listen on a free port number picked by the OS to avoid
rare `EADDRINUSE` errors on `SmartOS`.

Fixes: https://github.com/nodejs/node/issues/6197
PR-URL: https://github.com/nodejs/node/pull/6248


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>