unittest discovery and test.regrtest cannot find tests when they're not on the filesystem

Bug report

Bug description:

When the standard library is a zip archive on sys.path, the test package is not a directory: findtests() failed with an OSError from os.listdir() and load_package_tests() failed with "Start directory is not importable" from unittest discovery.

I'm still deciding if this should be done, it is perhaps odd thing to do to bundle tests into an archive before running, but this should apply to other import hooks as well and bundling tests into a zip is sort of a nice way of running tests against a fully zipped application. It's a feature regardless.

Linked PRs

• gh-157145

Commencez par examiner unittest discovery et test.regrtest, puis reproduisez l’échec avec la bibliothèque standard dans une archive ZIP sur sys.path. Le travail est considéré comme terminé lorsque test discovery et regrtest peuvent localiser et exécuter des tests via des hooks d’importation ne passant pas par le système de fichiers ; gh-157145 est le PR lié à examiner en premier.