test: add render test with pyppeteer - chromium based

[maartenbreddels]

Using pyppeteer (a python port of puppeteer) we let chrome render the page, make a screenshot, and compare it.

[maartenbreddels]

It does not run main.yml when it includes:

    - name: Upload artifacts (from test failures)
        uses: actions/upload-artifact@v1
        if: succeeded() || failed()
        with:
          name: artifacts
          path: artifacts

[maartenbreddels]

sometimes github just silently ignores incorrect yaml.

[maartenbreddels]

As mentioned puppeteer/puppeteer#661 it seems to be important to disable gpu rendering to avoid text rendering differences. Note that the pixel values differed by an absolute value of 1, so it's important to understand the difference, but we should probably expect more platform rendering differences.
We might want to have a tolerance of say 0.1 (10%) for passing tests?

[maartenbreddels]

Finding significant differences between the OS's:
Linux:

OSX:

(Note the H and l)

But these should pass when we allow a relative pixel difference of 10%.