Add wait in e2e when adding MR approvers

What does this MR do?

Adds a wait when searching MR approvers to solve an intermittent failure in the approval rules spec.

Closes #215876 (closed)

Screenshots

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