Add JSON string to `MockItemCompareService`

src/main/edu/wisc/services/cbs/item_comp/MockItemCompareService.php

@@ -19,7 +19,7 @@ class MockItemCompareService implements ItemCompareService
             $this->response = $response;
         } else {
             $this->response = new ItemCompareServiceResponse(
-                "Mock Item Compare Service Response"
+                '{"SKU": "12345", "attributes": [{"attribute_code":"ITEM_CATEGORY","cbs_value":"HARD/MP3Players/EARBUDS","mage_value":"SERV/PUB"}]}'
             );
         }
     }