[3.12] gh-121200: Log pwd entry in test_expanduser_pwd2() (GH-121207) (#121214)

gh-121200: Log pwd entry in test_expanduser_pwd2() (GH-121207)

Use subTest() to log the pwd entry in test_expanduser_pwd2() of
test_posixpath to help debugging.
(cherry picked from commit 05a6f8da6042cc87da1cd3824c1375d12753e5a1)

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