Update ellens-alien-game test to prevent returning an empty list from the factory

Before this change, you can pass the tests for `new_aliens_collection`
by returning an empty list. After this change, the returned list must
have the same length as the input list (so you create the same number of
Aliens as the input asks for).

Author: alexbird-hunterindustries

exercises/concept/ellens-alien-game/classes_test.py

@@ -199,6 +199,9 @@ def test_new_aliens_collection(self):
         test_data = [(-2, 6), (1, 5), (-4, -3)]
         actual_result = new_aliens_collection(test_data)
 
+        length_error_message = f"Expected {len(test_data)} Aliens, got {len(actual_result)}"
+        self.assertEqual(len(actual_result), len(test_data), msg=length_error_message)
+
         error_message = "new_aliens_collection() must return a list of Alien objects."
 
         for obj in actual_result: