add one more test case

Author: miguelHx

Python/chapter02/2.5 - Sum Lists/miguel_2.5_soln.py

@@ -426,6 +426,11 @@ def setUp(self):
                 LinkedList(),
                 LinkedList(),
                 LinkedList()
+            ),
+            (
+                LinkedList(1, 2, 3),
+                LinkedList(),
+                LinkedList(1, 2, 3)
             )
         ]