Add product structure to sandbox fixtures

The T-Shirts were shown as not available; this commit fixes it.

sites/sandbox/fixtures/child_products.json

         "date_created": "2013-12-12T16:33:57.426Z", 
         "product_options": [], 
         "slug": "oscar-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "parent"
     }
 },
 {
         "date_created": "2013-12-12T16:34:14.023Z", 
         "product_options": [], 
         "slug": "oscar-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "child"
     }
 },
 {
         "date_created": "2013-12-12T16:34:32.170Z", 
         "product_options": [], 
         "slug": "oscar-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "child"
     }
 },
 {
         "date_created": "2013-12-12T17:32:15.016Z", 
         "product_options": [], 
         "slug": "oscar-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "child"
     }
 },
 {
         "date_created": "2013-12-13T11:34:21.810Z", 
         "product_options": [], 
         "slug": "commandlinefu-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "parent"
     }
 },
 {
         "date_created": "2013-12-13T11:37:08.138Z", 
         "product_options": [], 
         "slug": "commandlinefu-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "child"
     }
 },
 {
         "date_created": "2013-12-13T11:37:45.834Z", 
         "product_options": [], 
         "slug": "commandlinefu-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "child"
     }
 },
 {
         "date_created": "2013-12-13T11:38:15.107Z", 
         "product_options": [], 
         "slug": "tangent-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "parent"
     }
 },
 {
         "date_created": "2013-12-13T11:38:49.769Z", 
         "product_options": [], 
         "slug": "tangent-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "child"
     }
 },
 {
         "date_created": "2013-12-13T11:39:12.859Z", 
         "product_options": [], 
         "slug": "tangent-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "child"
     }
 },
 {
         "date_created": "2013-12-13T11:39:53.073Z", 
         "product_options": [], 
         "slug": "tangent-t-shirt", 
-        "product_class": 1
+        "product_class": 1,
+        "structure": "child"
     }
 },
 {