1. Create a slot 'advtest' for later advancement.
select pg_create_logical_replication_slot('advtest', 'test_decoding');
2. Start a backend to create a slot (s) but block it before updating the
restart_lsn in ReplicationSlotReserveWal().
    select pg_create_logical_replication_slot('s', 'test_decoding');
3. start another backend to generate some new WAL files and advance the
slot (advtest) to the latest position but block it from updating the LSN in
XLogSetReplicationSlotMinimumLSN()
    select pg_switch_wal();
    select pg_switch_wal();
    SELECT pg_log_standby_snapshot();
    SELECT pg_log_standby_snapshot();
    select pg_replication_slot_advance('advtest', pg_current_wal_lsn());
    select pg_replication_slot_advance('advtest', pg_current_wal_lsn());
4. Release the backend to create slot (s).
5. execute checkpoint but block it before calling XLogGetReplicationSlotMinimumLSN()
6. release the advancement backend and then the LSN will be set to a new position.
7. release the checkpoint and the WALs required by the slot (s) are removed.