[3.12] gh-121200: Fix test_expanduser_pwd2() of test_posixpath (GH-121228) (#121231)

gh-121200: Fix test_expanduser_pwd2() of test_posixpath (GH-121228)

Call getpwnam() to get pw_dir, since it can be different than
getpwall() pw_dir.
(cherry picked from commit 02cb5fdee391670d63b2fc0a92ca9b36a32ac95a)

Co-authored-by: Victor Stinner <vstinner@python.org>