test: use dynamic port in test-dgram-send-callback-buffer

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

PR-URL: https://github.com/nodejs/node/pull/12942
Refs: https://github.com/nodejs/node/issues/12376


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <mhdawson@ibm.com>