* DEV: Add test case for syntax highlight of complex HTML
The commit 685e0da upgrade HighlightJS to version 11, which deprecates
syntax highlight of complex HTML elements. See https://github.com/highlightjs/highlight.js/issues/2889
This brought a regression of syntax highlighting of GitHub oneboxes,
which was fixed in 09cec7d. This commit adds a test case to prevent
future regressions like this one.
* fix test and warning
Some definitions rely on others, in particular the c/cpp/c-like ones,
and we were appending the bundle of all files in the folder.
Instead for testing I've limited us to just three definitions. This has
the benefit of being a lot smaller to download/parse in test mode too.
