clarified gradual guarantee in test

Author: randolf-scholz

stdlib/@tests/test_cases/builtins/check_callable.py

@@ -1,6 +1,8 @@
 from typing import Any
 
 
-def test_gradual_assignment(arg: Any) -> None:
+def check_gradual_guarantee(arg: Any) -> int:
+    # if Any materialized to `Callable[[], int] | None`, this function type checks
     if callable(arg):
-        _: int = arg()
+        return arg()  # OK
+    return -1