Re-work release end-to-end test

What does this MR do?

Re-work release end-to-end test to:

• remove unnecessary steps (like logging in twice, and logging out)
• make resources fabrication via API or GUI more explicit
• use before and after instead of before(:all) and after(:all)
• make make the code a little bit smaller
• improve test speed a bit

The re-worked test was executed five times, and it passed in every execution.

Note: This MR depends on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14524, so as soon as the dependency is merged this one will be rebased on top of master, and a force push will happen for it to be allowed to be merged too.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content