test: remove common.PORT from dgram test

Remove common.PORT from
test-dgram-send-callback-buffer-length-empty-address to eliminate
possibility that a dynamic port used in another test will collide
with common.PORT.

PR-URL: https://github.com/nodejs/node/pull/12944
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>

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

@@ -17,4 +17,6 @@ const onMessage = common.mustCall(function messageSent(err, bytes) {
   client.close();
 });
 
-client.send(buf, offset, len, common.PORT, onMessage);
+client.bind(0, () => client.send(buf, offset, len,
+                                 client.address().port,
+                                 onMessage));