Make tests skippable

Not the ideal solution considering how the CI yaml parser works and the lack of warnings that GitLab provides (not to mention this feature is not documented enough...), but since we might want to run a pipeline without waiting for the tests, we can now run a pipeline with CI_SKIP_TESTS="true".

This can be done through CI/CD -> Pipelines -> Run pipeline -> Select branch, add the variable, press run. This will override globally defined variables in the CI.