Add UI/visual regression tests

[krassowski]

Adds minimal visual regression tests for:

• Execution started at state
• Last executed at state
• Failed state (follow-up to Use execution failure timing information #100; to be enabled after lab 4.1 release)

The above tests will help to catch font size changes as these fixed by #103.

Adds tests for windowed notebook which is difficult to test without browser:

• a test for the new refined node attachment logic from Fix node attachment issue with windowed notebook #102
• a specific regression test adapting the reproducer from Lab@4 Bug Windowed Notebooks #101

The runtime of the entire UI tests suite is below 2 minutes.

[mlucool]

Both locally and on GH action I see a lot of things like:

[WebServer] [W 2023-11-04 23:51:33.682 ServerApp] wrote error: 'No such file or directory: tests-timing_outcomes-Timing-outcomes-Execution-started-at-state/Timing_outcomes.ipynb'
    Traceback (most recent call last):
      File "/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/tornado/web.py", line 1786, in _execute
        result = await result
                 ^^^^^^^^^^^^
      File "/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/jupyter_server/services/contents/handlers.py", line 121, in get
        model = await ensure_async(
                ^^^^^^^^^^^^^^^^^^^
      File "/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/jupyter_core/utils/__init__.py", line 189, in ensure_async
        result = await obj
                 ^^^^^^^^^
      File "/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/jupyter_server/services/contents/filemanager.py", line 863, in get
        raise web.HTTPError(404, "No such file or directory: %s" % path)
    tornado.web.HTTPError: HTTP 404: Not Found (No such file or directory: tests-timing_outcomes-Timing-outcomes-Execution-started-at-state/Timing_outcomes.ipynb)

I assume these are not expected. If so, can we remove this?

[krassowski]

I assume these are not expected. If so, can we remove this?

This is annoying indeed although not a problem with this PR - it also affects other other users of galata. I believe this is problem with jupyter-server and tracked in:

• Save-As should use a head request (it does not need the body) and get should catch non-existing. jupyterlab/jupyterlab#15153
• Don't let ContentsHandler's internal 404 bubble up. jupyter-server/jupyter_server#1328
  We can address these issues separately.