Update a test to avoid flakiness

What does this MR do?

Closes #37488 (closed).

This test has shown failures when running on CI, because automatic comments sometimes do not show up, but with the two other expectations, we can still test that the functionality of assigning multiple users to an issue works.

So, better having one less expectation, but having reliable test results, than having an extra expectation that leads to a flaky test and false negative results.

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