bpo-46426: Improve tests for the dir_fd argument (GH-30668) (GH-30739)

Ensure that directory file descriptors refer to directories different
from the current directory, and that src_dir_fd and dst_dir_fd refer
to different directories.

Add context manager open_dir_fd() in test.support.os_helper.
(cherry picked from commit 54610bb448a9cf5be77d53b66169fca4c11be6cb)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>