Add testable interface for datastore.

impl/prod/raw_datastore.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package prod
 
 import (
         ds "github.com/luci/gae/service/datastore"
         "github.com/luci/gae/service/info"
         "golang.org/x/net/context"
@@ skipping 137 matching lines @@
                 if !cb(dsR2F(k), tf.pm, cfunc) {
                         return nil
                 }
         }
 }
 
 func (d rdsImpl) RunInTransaction(f func(c context.Context) error, opts *ds.TransactionOptions) error {
         ropts := (*datastore.TransactionOptions)(opts)
         return datastore.RunInTransaction(d, f, ropts)
 }
+
+func (d rdsImpl) Testable() ds.Testable {
+        return nil
+}