test: adding common.mustCall() to callback

Adding the common.mustCall() to the callback as the test documentation
explains it.

test/parallel/test-cluster-dgram-reuse.js

@@ -39,10 +39,10 @@ function close() {
 // Creates the first socket
 const socket = dgram.createSocket({ type: 'udp4', reuseAddr: true });
 // Let the OS assign a random port to the first socket
-socket.bind(0, function() {
+socket.bind(0, common.mustCall(function() {
   // Creates the second socket using the same port from the previous one
   dgram.createSocket({ type: 'udp4', reuseAddr: true })
     .bind(socket.address().port, next);
   // Call next for the first socket
   next.call(this);
-});
+}));