Test Shutdown can be called multiple times.

src/google/protobuf/testing/googletest.cc

@@ -286,6 +286,8 @@ namespace {
 struct ForceShutdown {
   ~ForceShutdown() {
     ShutdownProtobufLibrary();
+    // Test to shutdown the library twice, which should succeed.
+    ShutdownProtobufLibrary();
   }
 } force_shutdown;