End-to-end test for multiple issue boards on group level

What does this MR do?

Closes gitlab-org/quality/testcases#225 (closed).

Note: I have deleted the original issue by mistake (https://gitlab.com/gitlab-org/quality/testcases/issues/178).

CE backport MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32644.

Note 2: besides adding the new test, some refactoring in the existing code base was also needed, together with the addition of new resources, and there's undoubtedly space for improvements, but I think this might already be a good first iteration.

Note 3: the scenario that adds, edits, and deletes an issue board will be addressed by this other issue.

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