Add direct test for the problem to make the fix commit clearer.
Noticed while implementing MkdirTemp on mac; the upstream tests for MkdirTemp fail without this.
File.Stat is left as a stub for now.
Tests are a bit stubbed down because os.ReadDir, os.Symlink, and t.TempDir are not yet (fully) implemented.
TODO: reimport tests from upstream as those materialize.