Make obtaining the Testable interface actually part of the interface.

impl/prod/taskqueue.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 (
         "fmt"
         "reflect"
 
@@ skipping 113 matching lines @@
 func (t tqImpl) Stats(queueNames []string, cb tq.RawStatsCB) error {
         stats, err := taskqueue.QueueStats(t.Context, queueNames)
         if err != nil {
                 return err
         }
         for _, s := range stats {
                 cb((*tq.Statistics)(&s), nil)
         }
         return nil
 }
+
+func (t tqImpl) Testable() tq.Testable {
+        return nil
+}