Mojo: Add a trivial end-to-end (in-process) test of Mojo shell.

mojo/services/test_service/test_service.mojom

Index: mojo/services/test_service/test_service.mojom
diff --git a/mojo/examples/window_manager/window_manager.mojom b/mojo/services/test_service/test_service.mojom
similarity index 67%
copy from mojo/examples/window_manager/window_manager.mojom
copy to mojo/services/test_service/test_service.mojom
index 213c5e5ceb5212b75a41ab2001763c12a273c74f..1065b20ab30847eca7a74f2ed21f5e31691f6ed4 100644
--- a/mojo/examples/window_manager/window_manager.mojom
+++ b/mojo/services/test_service/test_service.mojom
@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-module mojo {
+module mojo.test {
 
-interface IWindowManager {
-  CloseWindow(uint32 node_id);
+interface ITestService {
+  Ping() => ();
 };
 
-}
+}  // module mojo.test