GitLab CI Update, master branch (2021.11.24.)

In this MR I modernised the setup of the CI jobs. They are no longer making use of the build scripts of the repository, but rather just calling "simple" CMake/CTest commands directly.

Note that the CI setup does not work on my fork. 😦 (Still the old issue with not having access to the appropriate runner.) But all in all, I'm less and less convinced that a custom runner is needed for this repository at all. I think the authentication for the tests could be solved through "secret GitLab CI variables" as well. But we can discuss about those on a different forum.

@mavogel, please try in your branch if something like this would work on the main repository itself. (I'm not allowed to push branches into the main repository unfortunately.) The current issue with the test jobs not finding the test executables, I think, is an issue coming from the build scripts. Or if not, it would be an issue coming from the custom runner not working correctly. Whatever the case may be, moving to regular public runners would definitely make life a whole lot easier.