Improvements to make review_merge_request_spec more reliable

What does this MR do?

The test fails in staging about once in 10 times as it sometimes starts typing before the editor is fully loaded. As the result, we can get "ewed ReviText" instead of "Reviewed Text" which fails content expectations.

I added a necessary wait for the editor to load so that the typing is performed when it is stable.

See !28147 (closed) and gitlab-org/quality/testcases#705 (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