Test that notebooks can be trusted (2)

[mwouts]

This PR adds a test that a simple notebook can be trusted #797 .

Most of the problem with notebooks that cannot be trusted comes from using Jupyter Lab in a version prior to 3.0.13, in which cell ids were not available, so we recommend using a newer version for developing.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@94ac59d). Click here to learn what that means.
The diff coverage is 97.55%.

@@            Coverage Diff            @@
##             master     #826   +/-   ##
=========================================
  Coverage          ?   99.10%           
=========================================
  Files             ?      106           
  Lines             ?    10321           
  Branches          ?        0           
=========================================
  Hits              ?    10229           
  Misses            ?       92           
  Partials          ?        0           

Impacted Files	Coverage Δ
jupytext/reraise.py	25.00% <25.00%> (ø)
jupytext/kernels.py	92.85% <92.85%> (ø)
jupytext/pep8.py	95.58% <95.58%> (ø)
jupytext/cli.py	96.12% <96.07%> (ø)
jupytext/cell_reader.py	96.86% <96.49%> (ø)
jupytext/pandoc.py	96.61% <96.61%> (ø)
jupytext/cell_to_text.py	97.80% <97.16%> (ø)
jupytext/myst.py	97.19% <97.19%> (ø)
jupytext/config.py	97.28% <97.28%> (ø)
jupytext/contentsmanager.py	97.83% <97.70%> (ø)
... and 17 more

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