Add a topic on testing to the Dev Guide

[blaisep]

I recommended this to someone in the Sphinx Slack channel (we'll call him Kayce) as an example of how to write a theme. It turns out he already uses it however he found the tests a little hard to figure out.
So I looked at the dev guide:
https://pydata-sphinx-theme.readthedocs.io/en/stable/community/topics/manual-dev.html#manually-run-the-tests
and https://github.com/pydata/pydata-sphinx-theme/blob/main/tox.ini
and it doesn't describe how the tests are put together.

I almost seems like most of the heavy lifting happens in https://github.com/pydata/pydata-sphinx-theme/blob/main/tests/test_playwright.py#L2

Is this suggestion too vague?

[trallard]

Thanks for starting this discussion @blaisep.
I think this aligns with work we are already planning in #2171

Our test setup is more complicated than we'd like and needs some simplification/improvement. As part of this, I think a natural step would be to ensure the corresponding documentation is updated and that content gaps are addressed as needed.

[blaisep]

Thank you @trallard for the quick and encouraging reply. FWIW, a few people in the Slack suggested using @pradyunsg 's test
https://github.com/pradyunsg/sphinx-theme-builder/tree/main/tests
setup as an example

[trallard]

Oooo that is a good suggestion, I will check these as well.