Add User Testing interface to impl/memory.

service/user/testing.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
+
+// Testing is the interface that test implimentations will provide.
+type Testing interface {
+        // SetUser sets the user to a pre-populated User object.
+        SetUser(*User)
+
+        // Login will generate and set a new User object with values derived from
+        // email and admin values. It returns the generated clientID.
+        Login(email string, admin bool) (clientID string)
+
+        // Equivalent to SetUser(nil), but a bit more obvious to read in the code :).
+        Logout()
+}