Fix and capture test app build warnings

[jnikula]

Fix docutils warnings in app = Sphinx(). They're usually not visible, but when hitting other errors, pytest has captured the stderr and emits the warnings. Fix them, capture the stderr, ensure there are no errors with app = Sphinx() going forward, and compare the errors against expected errors like the parser tests do.

[jnikula]

The errors can be seen e.g. with make docker-test.fedora-37. It's hitting an issue with stdbool.h, but the logs have:

----------------------------- Captured stderr call -----------------------------
WARNING: while setting up extension sphinx.addnodes: node class 'toctree' is already registered, its visitors will be overridden
WARNING: while setting up extension sphinx.addnodes: node class 'desc' is already registered, its visitors will be overridden
WARNING: while setting up extension sphinx.addnodes: node class 'desc_signature' is already registered, its visitors will be overridden
WARNING: while setting up extension sphinx.addnodes: node class 'desc_signature_line' is already registered, its visitors will be overridden
WARNING: while setting up extension sphinx.addnodes: node class 'desc_content' is already registered, its visitors will be overridden
WARNING: while setting up extension sphinx.addnodes: node class 'desc_inline' is already registered, its visitors will be overridden

And it goes on and on.

[BrunoMSantos]

Nice fixes!