Visual Review Toolbar: Create Testing Plan

The Visual Review Toolbar is in the unique position of not being part of the Gitlab app. It is also vanilla Javascript. That means the testing story is still pretty unclear. While we are working on a single test to check that the script loads without throwing a runtime error (!28013 (closed)), we do need to make a more robust testing plan.

As the discussion in !28013 (closed) indicates, we could create an end-to-end suite to test the UI. We could also look at unit tests, once the script is modularized via !29433 (merged), although I would need a little help to figure out the best way to unit test imperative, non-library, DOM-heavy Javascript.