Add testing of the demo widget jupyter notebook

[dou-du]

Check this: https://blog.thedataincubator.com/2016/06/testing-jupyter-notebooks/ Notes:

• [ ] To check if it needs to compile something
• [ ] Might need to wait for the state to propagate back from the frontend to python.
• [ ] To check if there is an easy way to send interactions from the JS side

[giovannipizzi]

Mentioning @csadorf in case the link is useful also for the AiiDA lab testing