During the weekend I have discovered a bug on Rails activesupport gem. The truth is that it has already been reported to Rails. It is the issue https://github.com/rails/rails/pull/2472.
You can see my detailed answer here: http://stackoverflow.com/questions/7706487/rails-tests-showing-0-passed-when-all-pass/7939497#7939497
Use the gist that is referred there until the bug is fixed.
As always, your comments are welcome.