GTM standby test

Found that GTM synchronous backup does not work well.
What found:

1. Try to sync with GTM-Standby by gtm_sync_standby()
2. It does not return somehow.

Before these, called bkup_node_register_internal() to backup the command to GTM-Standby and it looks successful.

Maybe GTM-Standby does not handle backup messages correctly and GTM-Standby is waiting for some more information.