Difference in _use_fd_functions in shutil.py and test_shutil.py about cpython

[frenzymadness]

As I'm thinking about it more there might be a reason why this is calculated again in the tests and then compared with the value from the shutil module.

But given the current status, this check:

        _use_fd_functions = ({os.open, os.stat, os.unlink, os.rmdir} <=
                             os.supports_dir_fd and
                             os.listdir in os.supports_fd and
                             os.stat in os.supports_follow_symlinks)
        if _use_fd_functions:
            self.assertTrue(shutil._use_fd_functions)

in test_rmtree_uses_safe_fd_version_if_available checks whether both os.listdir and os.scandir are in os.supports_fd because that's the only difference between _use_fd_functions and shutil._use_fd_functions in this context.

Does it make sense to test this?

[frenzymadness]

Ccing @encukou since we were discussing it briefly.