Headless testing

[bollwyvl]

The most extensive headless browser testing is done by ipywidgets, with a simpler example from bqplot.

• [x] Does everything get loaded at runtime?
• [x] Can the sorter be opened?
• [x] Does the tour work?
• [x] Can the presenter be opened?
• [ ] Can the presentation be downloaded?

For testing pdf output, as with https://github.com/Anaconda-Server/nbbrowserpdf/issues/4 some kind of image-based diff should be done, but this can be deferred.

[bollwyvl]

Still need to add a proper download test, but ran into some issues with casper that i didn't want to hold this up.