End to end tests with Playwright

[jtpio]

Currently we use some JupyterHub fixtures and pytest for testing.

There is also #65 which uses the pytest-jupyterhub plugin to streamline testing even more.

It could be interesting to also do more end to end testing, like in the JupyterHub repo:

• https://github.com/jupyterhub/jupyterhub/blob/73b2307b367f8582e015fb6f8da5dd213523a943/jupyterhub/tests/browser/conftest.py#L5-L14
• https://github.com/jupyterhub/jupyterhub/blob/main/jupyterhub/tests/browser/test_browser.py