Add UI tests with Galata

[giswqs]

Galata is a set of helpers and fixtures for JupyterLab UI Testing. ipyleaflet and bqplot uses Galata for UI tests. We can add UI tests using Galata as well.

• https://github.com/jupyterlab/jupyterlab/tree/main/galata
• https://github.com/jupyter-widgets/ipyleaflet/tree/master/ui-tests
• https://github.com/bqplot/bqplot/tree/master/ui-tests