integrate unit tests

[alfredbez]

Is your feature request related to a problem? Please describe.
I found a problem in the codebase and wanted to provide a fix, but it's hard to do so without unit-tests.

Describe the solution you'd like
I want to add something like phpunit to make it possible to write and extend tests for this repository.

Describe alternatives you've considered
We could also integrate another testing solution like codeception or phpspec