Over in https://phabricator.services.mozilla.com/D185050 Katherine pointed out that it'd be nice to have easier helpers to run tests against the sidebar.

It would also help if we used a single helper from head.js that we could make a little more realistic over time - right now we're "just" assigning a mock data object directly to a sidebar implementation that we open without a corresponding shopping page actually being open next to it. It'd be nice if the tests were a bit more realistic, and for that too a single helper would be... helpful. Because rather than updating N tests you'd update the helper on its own.