Fix the testcase introduced in commit 81d20fbf7a.

Fix the testcase introduced in commit 81d20fbf7a.

The failed test was syncing failover replication slot to standby to test
that we remove such slots after the standby is converted to subscriber by
pg_createsubscriber.

In one of the buildfarm members, the sync of the slot failed because the
LSN on the standby was before the syncslot's LSN. We need to wait for
standby to catch up before trying to sync the slot with
pg_sync_replication_slots().

The other buildfarm failed because autovacuum generated a xid which is
replicated to the standby at some random point making slots at primary
lag behind standby during slot sync.

Both these failures wouldn't have occurred if we had used built-in
slotsync worker as it would have waited for the standby to sync with
primary but for this test, it is sufficient to use
pg_sync_replication_slots().

Reported-by: Alexander Lakhin as per buildfarm
Author: Kuroda Hayato
Reviewed-by: Amit Kapila
Backpatch-through: 17
Discussion: https://postgr.es/m/0dffca12-bf17-4a7a-334d-225569de5e6e@gmail.com
Discussion: https://postgr.es/m/OSBPR01MB25528300C71FDD83EA1DCA12F5DD2@OSBPR01MB2552.jpnprd01.prod.outlook.com