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

先檢查 unittest discovery 和 test.regrtest，然後使用 sys.path 上 ZIP 封存檔中的標準函式庫重現該失敗。完成的標準是 test discovery 和 regrtest 能夠透過非檔案系統匯入掛鉤定位並執行測試；gh-157145 是需要優先審查的關聯 PR。