Summary of Changes

A downside of our current Drone setup is the long build time for even the few tests we have right now. The tests currently take about 33 minutes to run completely, mainly because large parts of it are executed sequentially instead of in parallel. This PR tries to parallelize the system tests where possible. The codeception "build" step is executed just once and before all other system and api tests. Afterwards, all system and api tests are executed first with the minimal PHP version for the given RDBMS and then with the maximum PHP version. That means that different RDBMS versions are executed in parallel and the same RDBMS versions with different PHP versions are run sequentially.
To separate the tests properly from each other, the whole setup is copied each time from the common folder to a folder inside of each docker instance and the tests are then executed from there. That prevents the tests from interfering with each other.

Since the whole site has been copied from the common folder to the docker instances internal folder, the default _output folder for screenshots wont work. I tried to set the common folder as target again, but couldn't honestly test this yet, since the tests didn't fail for me...

The PHPUnit PHP8 patch which we added some time ago has been removed again, since it has been failing for months already and has been obsolete.