Add support for -tags to enable appengine testing.

go/src/infra/gae/apps/helloworld/app_test.go

 // Copyright 2014 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.
 
+// +build appengine
+
 package helloworld
 
 import (
         "testing"
 
         "appengine/aetest"
         "appengine/memcache"
         "appengine/user"
 
         . "github.com/smartystreets/goconvey/convey"
@@ skipping 19 matching lines @@
                         Key:   "some-key",
                         Value: []byte("some-value"),
                 }
                 err = memcache.Set(c, it)
                 So(err, ShouldBeNil)
                 it, err = memcache.Get(c, "some-key")
                 So(err, ShouldBeNil)
                 So(it.Value, ShouldResemble, []byte("some-value"))
         })
 }