test: remove arrow function (for now)

It was included by the previous commit, but the test code is not
currently transpiled to ES5 so it breaks IE in the CI.

Let's use a plain function for now.

test/connection.js

@@ -458,7 +458,7 @@ describe('connection', function () {
 
       var socket = manager.socket('/room1');
 
-      setTimeout(() => {
+      setTimeout(function () {
         manager.socket('/room2');
       }, delay);
     });