test: bind to 0 in dgram-send-callback-buffer-length

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

PR-URL: https://github.com/nodejs/node/pull/12943
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.js

@@ -38,4 +38,7 @@ const messageSent = common.mustCall(function messageSent(err, bytes) {
   client.close();
 });
 
-client.send(buf, offset, len, common.PORT, '127.0.0.1', messageSent);
+client.bind(0, () => client.send(buf, offset, len,
+                                 client.address().port,
+                                 '127.0.0.1',
+                                 messageSent));