Add composer test script

This MR brings one step closer to running the tests in a standard way by adding a composer test command. In order to work, a few include and require had to be updated to use relative file path rather than relative command directory path. All the unit tests pass.

Currently the command composer test has to be run from within the vendor_bundled. Ultimately it will be ran at the root. The command phpunit will be available also at a later stage.