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: #12929
Refs: #12376
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-dgram-send-callback-buffer-empty-address.js

@@ -14,4 +14,4 @@ const onMessage = common.mustCall(function(err, bytes) {
   client.close();
 });
 
-client.send(buf, common.PORT, onMessage);
+client.bind(0, () => client.send(buf, client.address().port, onMessage));