Introduce E2E tests and CI integration

[Mgsy]

• [x] Prepare a set of tests that covers:
  • Cloning the repository,
  • Opening the demo directory,
  • Installing dependencies,
  • Running the server,
  • Verifying if the demos work
• [x] Integrate tests with CI, so each PR would be tested

[mlewand]

At the very least it should check whether the editor is created and not crashed (as it could be a config issue).

And also a CRON that runs it at least once a day. Think of when CKE5 releases and there's duplicated module error or something like that.