Fix BF introduced in commit ddd5f4f54a.

Fix BF introduced in commit ddd5f4f54a.

The failure is that the remote slot is not synchronized after the same
slot on standby gets invalidated. The reason was that remote_slot's
restart_lsn was lagged behind the standby's oldest WAL segment. The test
didn't ensure that remote_slot's LSN was advanced to the latest position
before we tried to sync the slots via new the function
pg_sync_replication_slots().

Discussion: https://postgr.es/m/CAA4eK1JLBi3HzenB6do3_hd78kN0UDD1mz-vumWE52XHHEq5Bw@mail.gmail.com