[3.11] pythongh-91387: Fix tarfile test on WASI (pythonGH-93984)

WASI's rmdir() syscall does not like the trailing slash.
(cherry picked from commit dd78aae)

Co-authored-by: Christian Heimes <christian@python.org>

Lib/test/test_tarfile.py

@@ -1031,7 +1031,7 @@ def test_longname_directory(self):
                     os.mkdir(longdir)
                     tar.add(longdir)
                 finally:
-                    os.rmdir(longdir)
+                    os.rmdir(longdir.rstrip("/"))
             with tarfile.open(tmpname) as tar:
                 self.assertIsNotNone(tar.getmember(longdir))
                 self.assertIsNotNone(tar.getmember(longdir.removesuffix('/')))