dbus-ping: reduce overhead when creating messages

calling dbus_message_get_type(), dbus_message_get_path(),
dbus_message_get_interface() and dbus_message_get_member() on each
iteration might get expensive.
Since the dbus-test-service uses only dbus_message_get_sender()
internally to produce a reply message, this makes both sides more
symmetrical.
to
produce the