sphinx.testing is undocumented

https://github.com/sphinx-doc/sphinx-testing is marked deprecated and points to sphinx.testing as a replacement. However, the only documentation for this is https://www.sphinx-doc.org/en/master/devguide.html#unit-testing, which does not provide any details.

My first assumption was that maybe it's sufficient to replace from sphinx_testing import with_app with from sphinx.testing import with_app, but that does not seem to be the case:

ImportError: cannot import name 'with_app' from 'sphinx.testing'

I can sort of figure out the use of sphinx.testing by looking at the source code for Sphinx' own tests, but nonetheless there should be proper documentation. The (closed) issue #3458 already pointed out that documentation is still a todo item.

Start with the devguide unit-testing page and inspect the Sphinx project's own test sources to understand the sphinx.testing API and its documented entry points. Document the replacement for sphinx-testing, including the supported usage and the limitations demonstrated by the failed with_app import, so users can follow the documentation without reading the source.