Add missing holdouts and region fields to datafile expected response

The actual datafile response from the SDK includes 'holdouts' and 'region'
fields that were missing from our expected response.

Author: Mat001

tests/acceptance/datafile.py

@@ -240,8 +240,10 @@
         }
     ],
     "groups": [],
+    "holdouts": [],
     "integrations": [],
     "projectId": "16931203314",
+    "region": "US",
     "revision": "139",
     "rollouts": [
         {