[3.12] gh-109295: Fix test_os.test_access_denied() for TEMP=cwd (GH-109299) (#109304)

gh-109295: Fix test_os.test_access_denied() for TEMP=cwd (GH-109299)

Fix test_os.test_access_denied() when the TEMP environment variable
is equal to the current working directory. Run the test using a
different filename, since self.fname already exists in this case.
(cherry picked from commit 7dedfd36dc16d9e1e15d7d0b0a636dd401a5a543)

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