Try to avoid 'HeadlessException: No X11 DISPLAY' on GitHub Actions

Try to avoid 'HeadlessException: No X11 DISPLAY' on GitHub Actions

We have 84 times this kind of exceptions in 'ajde' tests in our build
logs on GitHub, even though the tests seem to pass:

  HeadlessException: No X11 DISPLAY variable was set,
  but this program performed an operation which requires it.

I found this discussion:
https://github.com/juliangruber/browser-run/issues/147

And then this GitHub Action:
https://github.com/marketplace/actions/gabrielbb-xvfb-action

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>