Split quarantined test that does too much into smaller and more isolated test cases

What does this MR do?

Closes gitlab-org/quality/staging#19 (closed), based on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8942#note_177276196.

If this MR is accepted, then this other one needs to be closed without merging: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8942.

Note: The old test case was split into five test cases, but because commenting (on issues and epics) is flaky, two of the new tests were quarantined. As soon as this MR is merged, issues will be opened to address the new quarantined tests, and then the issue links will be added as comments before the tests.

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