JavaScript tests

[mgeisler]

Today, we don't have any tests for the JavaScript code found in

• https://github.com/google/comprehensive-rust/blob/main/theme/book.js: executes the Rust Playground code (comes from the upstream mdbook project).
• https://github.com/google/comprehensive-rust/blob/main/theme/speaker-notes.js: our own home-grown speaker note system.
• https://github.com/google/comprehensive-rust/blob/main/theme/save-playgrounds.js: not used currently, could save the state of a playground between page loads.

This has caused bugs in the past (the playgrounds disappeared at some point) and it makes it hard to upgrade mdbook (because we need to merge in changes to book.js manually).

It would be very helpful to have tests for these files. This should be something we can run on the command line in GitHub CI. I think we need something which

• We can run on pre-rendered HTML files. I envision us running mdbook build (for each language).
• A headless browser would then be needed to inspect the pages.
• We would need a unit testing framework of some sort.
  • I used https://www.protractortest.org/ in the past, but there are many others.