test: revert arrow function for echoServer.on

Author: Shubhamurkade

test/parallel/test-net-persistent-ref-unref.js

@@ -29,7 +29,7 @@ TCPWrap.prototype.unref = function() {
 
 echoServer.listen(0);
 
-echoServer.on('listening', () => {
+echoServer.on('listening', function() {
  const sock = new net.Socket();
  sock.unref();
  sock.ref();