GPL-2.0 in test tests/php

[OliverFendt]

Hi, thank you very much for providing this package. I am currentliy doing a license analysis of markdown and came across the GPL-2.0 license text in tests/php. How is the relation of the GPL-2.0 to the files in this repo, does it apply to any of the files, if yes it would be very useful that those files contain a reference to the GPL-2.0 otherwise no one can determine to which files it applies and so it will be very hard to comply to the license.