Automated widget testing

[kylebarron]

There's now pytest-ipywidgets.

See https://github.com/jupyter-widgets/ipywidgets/issues/2934#issuecomment-2109499356. Note that this might still be quite difficult to implement because we need to emulate a GPU as well if we want to render data. I believe deck used some setup with xvfb to emulate the GPU in CI