test(harness): Add e2e testing harness using playwright

[nicojs]

Add a e2e testing harness using playwright. Also add 1 demo scenario using the revealjs demo presentation

To add more e2e tests, add more scenarios to the test/input directory. New scenarios found there are automatically picked up.

• Add a simple webserver implementation to serve presentations and pdf files using koa, a small modern web server.
• Add a test-prepare-pdfs script to generate pdfs based on the scenarios in the test/input directory.
• Add a test script to open the pdfs with playwright using a known workaround
  • Verify each page in each pdf using snapshot tests
• Add a ci workflow that runs for each PR and commit, as well as scheduled each week.
• Add a update-snapshots workflow that can be triggered manually. It needs a GH_TOKEN secret, which you can generate via the github UI

Closes #253

[nicojs]

I see that the demo has a lot of external images. This can be a problem because the tests could fail because of a change on their end. Maybe it's better to pull them into the input/revealjs-demo directory? What do you think @astefanutti ?

[nicojs]

It turned out to be not a lot of files, so I went ahead and did it.

[nicojs]

That's actually a great idea, I wasn't thinking about that. It would mean a manual "update-snapshot" process, which I can automate for you in the update-snapshots workflow.

[astefanutti]

That's actually a great idea, I wasn't thinking about that. It would mean a manual "update-snapshot" process, which I can automate for you in the update-snapshots workflow.

That manual "update-snapshot" would be perfectly ok. Would you rather prefer to have that PR merged as is, or keep it open until we move to using PDFs as the source of truth? I'm fine either ways 👍🏼.

[astefanutti]

Let's merge this. We can look into using PDFs as the source of truth later. Thanks again!