Simplify flaky MR review QA test

What does this MR do?

The test duplicated coverage provided by unit/feature tests, so this change makes the test much simpler, covering only the happy path of the MR review feature, plus additional coverage for an S1 regression.

• Adds waits after a few actions so that the test doesn't continue to another action while the previous is still not complete.
• Sets minimum when finding elements by index because we know at least how many elements to expect. And then we don't need an extra wait before using all because minimum takes care of that - it will no longer return an empty array if no elements are found, it will raise an exception instead.

The test is failing in master and isn't quarantined, so Pick into auto-deploy

Closes #35797 (closed)

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers