Fix unstable test in 040_pg_createsubscriber.

The slot synchronization failed because the local slot's (created during
slot synchronization) catalog_xmin on standby is ahead of remote slot.
This happens because the INSERT before slot synchronization results in the
generation of a new xid that could be replicated to the standby. Now
before the xmin of the physical slot on the primary catches up via
hot_standby_feedback, the test has created a logical slot that got some
prior value of catalog_xmin.

To fix this we could try to ensure that the physical slot's catalog_xmin
is caught up to latest value before creating a logical slot but we took a
simpler path to move the INSERT after synchronizing the logical slot.

Reported-by: Alexander Lakhin as per buildfarm
Diagnosed-by: Amit Kapila, Hou Zhijie, Alexander Lakhin
Author: Hou Zhijie
Backpatch-through: 17
Discussion: https://postgr.es/m/bde6ac67-69cc-c104-5ab6-dd4f5deadf24@gmail.com

src/bin/pg_basebackup/t/040_pg_createsubscriber.pl

@@ -227,10 +227,6 @@ command_fails(
 	],
 	'primary server is in recovery');
 
-# Insert another row on node P and wait node S to catch up
-$node_p->safe_psql($db1, "INSERT INTO tbl1 VALUES('second row')");
-$node_p->wait_for_replay_catchup($node_s);
-
 # Check some unmet conditions on node P
 $node_p->append_conf(
 	'postgresql.conf', q{
@@ -306,6 +302,14 @@ my $result = $node_s->safe_psql('postgres',
 );
 is($result, 'failover_slot', 'failover slot is synced');
 
+# Insert another row on node P and wait node S to catch up. We
+# intentionally performed this insert after syncing logical slot
+# as otherwise the local slot's (created during synchronization of
+# slot) xmin on standby could be ahead of the remote slot leading
+# to failure in synchronization.
+$node_p->safe_psql($db1, "INSERT INTO tbl1 VALUES('second row')");
+$node_p->wait_for_replay_catchup($node_s);
+
 # Create subscription to test its removal
 my $dummy_sub = 'regress_sub_dummy';
 $node_p->safe_psql($db1,