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

先检查 unittest discovery 和 test.regrtest，然后使用 sys.path 上 ZIP 归档中的标准库重现该失败。完成的标准是 test discovery 和 regrtest 能够通过非文件系统导入钩子定位并运行测试；gh-157145 是需要首先审查的关联 PR。