Add e2e test case for testing GitLab Pages

* GitLab Pages has been set up on Staging and Production projects.
Testing that GitLab Pages is reachable in these private projects
by logging in through the Helmholtz AAI.
* Refactor and extract function to log in into Helmholtz AAI.
4 jobs for 17-test-case-with-gitlab-pages in 2 minutes and 59 seconds (queued for 1 second)