Add tests that use currently unused MR review page object methods

In gitlab-org/gitlab!21606 (merged) I removed some steps from a test to simplify it and fix flakiness.

However, I neglected to remove the page object methods and frontend code that was no longer used.

Proposal

Add coverage back in a separate test and try to ensure it's not flaky.

Include more coverage for common workflows involving diff comments (see gitlab-org/gitlab#235966 (closed))