test: fix test-timers.reliability on OS X

Fixes: https://github.com/nodejs/node/issues/4404
PR-URL: https://github.com/nodejs/node/pull/5379
Reviewed-By: Julien Gilli <jgilli@fastmail.fm>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

test/timers/test-timers-reliability.js

@@ -32,7 +32,7 @@ var intervalFired = false;
  */
 
 var monoTimer = new Timer();
-monoTimer.ontimeout = function() {
+monoTimer[Timer.kOnTimeout] = function() {
     /*
      * Make sure that setTimeout's and setInterval's callbacks have
      * already fired, otherwise it means that they are vulnerable to