Frontend tests

[domna]

I don't know yet how to test it properly, there are different frontend tests available. ipywidgets uses karma to test their widgets, which is just a test runner so an actual testing suite is needed. There is also https://github.com/robots-from-jupyter/robotframework-jupyterlibrary available, which seems to be an easy regression based approach.

• Get test data
• Decide on a test suite (frontend tests?? + for functional tests pytest)