Basic unit tests stuff

[trasher]

Unit testing a GLPi plugin is not really difficult, but it requires some basic work before.

FusionInventory plugin has unit tests, but the whole is based on ant, and I would prefer not to use this. formcreator also now have tests; maybe it would be interesting to take an eye on it.

For testing framework... Logically, we should use PHPUnit, as this is used in GLPi, even if I really would prefer to work with atoum.