Add User Testing interface to impl/memory.

service/user/testable.go

Index: service/user/testable.go
diff --git a/service/user/testable.go b/service/user/testable.go
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+++ b/service/user/testable.go
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+// 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
+
+// Testable is the interface that test implimentations will provide.
+type Testable 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 clientID, and admin values. If clientID is provided, the User will
+	// look like they logged in with OAuth. If it's empty, then this will look
+	// like they logged in via the cookie auth method.
+	Login(email, clientID string, admin bool)
+
+	// Equivalent to SetUser(nil), but a bit more obvious to read in the code :).
+	Logout()
+}