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>

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

@@ -14,4 +14,7 @@ const onMessage = common.mustCall(function(err, bytes) {
   client.close();
 });
 
-client.send(buf, common.PORT, common.localhostIPv4, onMessage);
+client.bind(0, () => client.send(buf,
+                                 client.address().port,
+                                 common.localhostIPv4,
+                                 onMessage));