Add composer test script

I'm working on lowering the entry barrier point for new collaborators by making it more standard to run the tiki tests. This adds a composer test command. Currently it still has to be run from within the non-standard vendor_bundled folder. I will see how to add it from the root, and add the possibility to run directly phpunit later on,

To be able to run the tests from any path, I've changed many requires and includes to work with files path rather than path from where the script is launched. That is also an improvement, and ultimately we should get rid of those include/require. That's for a next step. All the unit tests pass as before I made the changes. I've check a bit through the UI, seems ok too.