Add a test case for intermediate-generic

Author: laike9m

challenges/intermediate-generic/question.py

@@ -15,3 +15,4 @@ def add(a, b):
 assert_type(add(1, 2), int)
 assert_type(add("1", "2"), str)
 assert_type(add(["1"], ["2"]), List[str])
+assert_type(add(1, "2"), int)  # expect-type-error