create headless testing for the package

[satra]

using chrome headless via docker, we can create test notebooks for the package.

[janfreyberg]

not sure how this would work, since I don't know what exactly you could test with that that you can't test otherwise - would this allow testing of the exact javascript state of a widget after creation?

[arokem]

This looks like a potential way forward: https://bokeh.pydata.org/en/latest/docs/user_guide/export.html