[1.2] Fix doc about running unit tests

Alternative to #1075.

I've kept the syntax using phpunit.phar as it must be the most portable way to run PHPUnit (like it is for Composer).
It's not that I want to start an argument, but how do you think this would work, with `phpunit.phar` not even existing in the root of the project?

![its-magic-shia-labeouf-gif](https://cloud.githubusercontent.com/assets/605483/5443279/aac55c82-84a0-11e4-9b9c-5a01edbca135.gif)

You should probably add some notes about how to install PHPUnit globally, then, if you still think it is a good idea.