Add service_test GN template

services/service_manager/public/cpp/test/service_test_catalog.h

+// Copyright 2017 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.
+
+namespace service_manager {
+namespace test {
+
+// This symbol must be defined by any target linking against the
+// ":run_all_service_tests" target in this directory. Use the service_test
+// GN template defined in
+// src/services/service_manager/public/tools/test/service_test.gni to
+// autogenerate and link against a definition of the symbol dervied from the
+// contents of a generated service catalog. See the service_test.gni
+// documentation for more details.
+extern const char kServiceTestCatalog[];
+
+}  // namespace test
+}  // namespace service_manager