Add User Testing interface to impl/memory.

service/user/interface.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 user
 
 // Interface provides access to the "appengine/users" API methods.
 type Interface interface {
         Current() *User
         CurrentOAuth(scopes ...string) (*User, error)
 
         IsAdmin() bool
 
         LoginURL(dest string) (string, error)
         LoginURLFederated(dest, identity string) (string, error)
         LogoutURL(dest string) (string, error)
 
         OAuthConsumerKey() (string, error)
+
+        // If this implementation supports it, this will return an instance of the
+        // Testable object for this service, which will let you 'log in' virtual users
+        // in your test cases. If the implementation doesn't support it, it will
+        // return nil.
+        Testable() Testable
 }