Add unit-tests to PDS Interop maintained dependencies/packages

[Potherca]

We want to have more unit-level test in all PDS Interop PHP repositories that are dependencies for this repository.

This ticket serves as full list of all the steps that need to be taken.

All repositories (except solid-nextcloud) already have real or dummy tests and a GitHub Action to run those test(s).

Per repository, a separate ticket has been created outlining what should be tested in more details:

• php-solid-auth Add (more) unit-tests php-solid-auth#25
• php-solid-crud Add (more) unit-tests php-solid-crud#21
• flysystem-nextcloud Add (more) unit-tests flysystem-nextcloud#10
• flysystem-rdf Add (more) unit-tests flysystem-rdf#11