> while running the test command (pysetup run test)?
Yes, that’s the correct invocation.

> The use case: [...]
I agree with the need.  I’ve added a dep on the bug requesting the addition of a build_distinfo command; once it is done, this bug will be solved, since test runs build and places the build lib in sys.path (which also enables running tests on code converted by 2to3 at build time).

> The function that finds the configuration uses sysconfig paths
> (sysconfig.get_paths) which rely on the packaging database (PEP 376
> API now in packaging.database).
sysconfig does rely on packaging.

> To find the default config, or resource in packaging terms, I'm using
> the packaging.database.get_file_path, which requires the distribution
> metadata (dist-info) be available within the Python path
This however is correct.

> I'd like to know where this RESOURCES file came from? I don't recall,
> nor see mention of it in PEP 376.
This is the implementation of the discussion originally started in https://bitbucket.org/tarek/distutils2/src/6c3d67ed3adb/docs/design/wiki.rst  I think it was originally intended to be a new PEP, but during distutils2 sprints it was directly coded.  Resources replace data_files.

> I would also have expected to find something in
> packaging.command.install_distinfo command related to this file.
It’s subtle; grep for RESOURCES and you’ll find it.  The list of files is built by install_data and written by install_distinfo.  If you want to learn more about this code, go to a distutils2 repo and look at the log.