Make obtaining the Testable interface actually part of the interface.

service/taskqueue/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 taskqueue
 
 import (
         "github.com/luci/luci-go/common/errors"
         "golang.org/x/net/context"
 )
@@ skipping 57 matching lines @@
         if err == nil {
                 err = lme.Get()
         }
         return ret, err
 }
 
 func (t *taskqueueImpl) Raw() RawInterface {
         return t.RawInterface
 }
 
+func (t *taskqueueImpl) Testable() Testable {
+        return t.RawInterface.Testable()
+}
+
 var _ Interface = (*taskqueueImpl)(nil)
 
 func Get(c context.Context) Interface {
         return &taskqueueImpl{GetRaw(c)}
 }