test: use arrow syntax for anonymous callbacks

PR-URL: #24691
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

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

@@ -4,7 +4,7 @@ const assert = require('assert');
 const net = require('net');
 const TCPWrap = process.binding('tcp_wrap').TCP;
 
-const echoServer = net.createServer(function(conn) {
+const echoServer = net.createServer((conn) => {
   conn.end();
 });
 
@@ -32,7 +32,7 @@ echoServer.on('listening', function() {
   sock.unref();
   sock.ref();
   sock.connect(this.address().port);
-  sock.on('end', function() {
+  sock.on('end', () => {
     assert.strictEqual(refCount, 0);
     echoServer.close();
   });