Hide spurious stderr output in notebook_cmd_test.py

[user202729]

If you look at the log on the CI, without this change, it would print out some stuff on stderr.

============================= test session starts ==============================
platform linux -- Python 3.12.11, pytest-8.4.1, pluggy-1.6.0
rootdir: /home/runner/work/sage/sage
configfile: pyproject.toml
plugins: xdist-3.8.0
collected 76 items / 2 skipped

src/sage/cli/eval_cmd_test.py ..
usage: pytest [-h] [-n [{jupyter,jupyterlab}]]
pytest: error: argument -n/--notebook: invalid choice: 'invalid' (choose from jupyter, jupyterlab)
src/sage/cli/notebook_cmd_test.py .....
src/sage/doctest/parsing_test.py ...........

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[github-actions]

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[tobiasdiez]

Thanks, but using capsys is a bit cleaner/stable and also allows to verify that the correct error is written to stderr.

[tobiasdiez]

Thanks!