[test] Fix nits

Author: lpinca

test/receiver.test.js

@@ -1126,7 +1126,7 @@ describe('Receiver', () => {
     const receiver = new Receiver();
     let count = 0;
 
-    receiver.on('message', function () {
+    receiver.on('message', () => {
       if (++count === 2) {
         throw new Error('Oops');
       }

test/websocket.test.js

@@ -3109,7 +3109,7 @@ describe('WebSocket', () => {
         socket.write(Buffer.from('foo\r\n'));
       });
 
-      server.listen(0, function () {
+      server.listen(0, () => {
         const ws = new WebSocket(`ws://localhost:${server.address().port}`);
 
         ws.on('open', () => done(new Error("Unexpected 'open' event")));