Issue #25911: Backport os._DummyDirEntry fixes

* Fix test_os.BytesWalkTests on Windows
* Mimick better the reference os.DirEntry on Windows
* _DummyDirEntry now caches os.stat() result
* _DummyDirEntry constructor now tries to get os.stat()
* Fix os._DummyDirEntry.is_symlink(), don't follow symbolic links:
  use os.stat(path, follow_symlinks=False).