Refactor unit test to allow to debug them in RubyMine/IntelliJ

**Debugging** individual test is really painful with RubyMine/Intellij as `require test_helper` cannot be loaded. With this PR where `require test_helper` have been replaced by ` require "#{File.dirname __FILE__}/test_helper"`, we can run unit test :  

![screen shot 2014-02-03 at 12 30 12](https://f.cloud.github.com/assets/463790/2064175/9c334292-8cc6-11e3-907c-cfd5b4ac2453.png)

**REMARK** :  rake test can be run without any issues

https://gist.github.com/8782396