[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).
If you follow composer's install instructions, you get the `composer.phar`.
https://getcomposer.org/download/
```
curl -sS https://getcomposer.org/installer | php
```

The same for PHPUnit:
https://phpunit.de/manual/current/en/installation.html

```
wget https://phar.phpunit.de/phpunit.phar
php phpunit.phar --version
```