Third tentative of removing flakiness from 'promote issue into epic' e2e test

What does this MR do?

Closes https://gitlab.com/gitlab-org/quality/nightly/issues/106.

The first and second tentatives can be found below:

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13977
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13913

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