tests/basics/builtin_slice: Add test for "slice" builtin name.

tests/basics/builtin_slice.py

@@ -4,4 +4,8 @@
 class A:
     def __getitem__(self, idx):
         print(idx)
-A()[1:2:3]
+        return idx
+s = A()[1:2:3]
+
+# check type
+print(type(s) is slice)