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

[gpshead]

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-157144: Let unittest discovery and regrtest find tests inside archives on sys.path #157145