test: replace common.PORT in dgram test

Replace common.PORT with available port assigned by the operating
system in test-dgram-send-callback-buffer-empty-address.

PR-URL: https://github.com/nodejs/node/pull/12929
Refs: https://github.com/nodejs/node/issues/12376


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>