End to end testing with Galata

[jtpio]

The end to end testing in retrolab was added before galata was open sourced: https://github.com/jupyterlab/galata

We should ideally update to using galata, similar to jupyterlab/jupyterlab#10251

This might also help consolidate galata itself so it can be use more widely on various JupyterLab distributions.

[jtpio]

There are now UI Tests with playwright: https://github.com/jupyterlab/retrolab/tree/main/app/test

#228 tracks adding more UI Tests. We should check there whether we are able to use the galata helpers.