Add UI tests

[jtpio]

UI tests would open all the demo notebooks and make sure all cells execute fine without any error.

This would help more easily catch issues like #69 (comment)

[jtpio]

Thanks @martinRenou for adding more UI tests in jupyterlite/xeus#276 👍

Do you think it could be worth adding similar tests here in that repo?

That could help bring more confidence the build is passing fine and make merging PRs a bit easier.

[jtpio]

That could help bring more confidence the build is passing fine and make merging PRs a bit easier.

Because even if the UI tests pass on the jupyterlite-xeus repo, it doesn't necessarily mean there won't be any issues here if we update the environment, because new packages may be picked up during the new build and may be having issues?