bpo-35994: add sub dir for sub2_tree in os.walk test if symlink is not supported (GH-11853)

https://bugs.python.org/issue35994

Lib/test/test_os.py

@@ -923,7 +923,7 @@ class WalkTests(unittest.TestCase):
                               ["broken_link", "broken_link2", "broken_link3",
                                "tmp3"])
         else:
-            self.sub2_tree = (sub2_path, [], ["tmp3"])
+            self.sub2_tree = (sub2_path, ["SUB21"], ["tmp3"])
 
         os.chmod(sub21_path, 0)
         try: