Add more UI tests

[jtpio]

Problem

For now there are a couple of UI tests running on both chromium and firefox.

They are useful as basic smoke tests, and to check RetroLab opens activities in new browser tabs.

We should cover more features specific to RetroLab in these tests.

Proposed Solution

The existing end to end tests are located in this folder: https://github.com/jupyterlab/retrolab/tree/main/app/test

We could follow the galata setup in JupyterLab: https://github.com/jupyterlab/jupyterlab/tree/master/galata

And try to use Galata with Retro, or stick to plain Playwright like it is now.

Additional context

This could help catch regressions like #226.

[jtpio]

These tests could cover:

• the loading of federated extensions (to avoid issues like Fix handling of federated extensions #245)
• visual regression
• RTC

[jtpio]

This will also be useful to catch missing plugins from upstream when we update to newer packages, like in #263

[jtpio]

Closing in favor of jupyter/notebook#6322