Add test consent requirements of sideloaded EPUB publications

[GJFR]

Add test for checking whether sideloaded EPUB publications require user consent for scripting and loading of remote resources (#297, spec).

• I split this test into _scripting and _network, but of course, these can be merged if that would be more convenient.
• There is a plethora of ways to instantiate remote connections, but to keep it simple I only included the resource types that are explicitly mentioned in the spec.
  • I used an audio file hosted by MDN, because I couldn't find one hosted by W3C.
  • In my opinion, we should include a remote script as well, however, I could not find a suitable one hosted by W3C. Is there a possibility to host a small script on the W3C domain?

[GJFR]

Perfect, thanks! This script should be sufficient:

function test_remote_scripting_support() {
  const report = document.getElementById('remote_scripting_support');
  report.textContent = 'This reading system fetched and executed a remote script.';
}
window.addEventListener('load', test_remote_scripting_support);

[GJFR]

Currently we check whether stylesheets are fetched and loaded without consent by simply referring to the spec's stylesheet, but this is not ideal (and this stylesheet may be subject to change in the future). Could you please host the stylesheet below as well? I think this would make the tests more future-proof and robust.

.paint_red {
  color: red
}

It's the last one, I promise 😉

[GJFR]

All done!