[3.13] Clean up test_posixpath (GH-134315) (GH-134317)

* Ensure that created files and dirs are always removed after test.
  Now addCleanup() does not conflict with tearDown().
* Use os_helper.unlink() and os_helper.rmdir().
* Import TESTFN from os_helper.
(cherry picked from commit e29171bf8a26b5faf97222e07a7d5f33c9eb272b)

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