Backfill system tests

[parente]

@Lull3rSkat3r did a nice job getting a test harness all setup to do UI system tests. He even seeded a set of utilities and basic examples. But we didn't want to spend the cycles at that time improving the test coverage over all the features.

Now that the UX for layout and preview within the notebook is settling down, it might be worthwhile backfilling the system test suite in a few areas.

• [ ] Show/hide code on hover
• [ ] More info link
• [ ] Menu items for adding / removing all cells
• [ ] Download menu option (confirm click works at least)
• [ ] Deploy local menu option (confirm click works at least)