Adjust epic management end-to-end test

What does this MR do?

This adjustment is needed due to a feature change. Now, the way we relate issues to epics has changed, and so the test needed to be updated accordingly.

This change is related to the following failure in the following nightly pipeline: https://gitlab.com/gitlab-org/quality/nightly/-/jobs/347826845

This MR is related to #35784 (closed), and by merging it we might be able to even close the issue.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content