Support a single-file html figure test report

[Cadair]

GitHub actions now supports non-zipped artifacts but unlike circle ci it only works for a single file.

I think it should be technically possible to inline all of the js, css and figures into one (massive 😆) html page?

[astrofrog]

We could just run https://github.com/Y2Z/monolith as a final step? Not sure if it will work with the option to download the hashes though.