Close yet another race condition in replication slot test code

Buildfarm shows that this test has a further failure mode when a
checkpoint starts earlier than expected, so we detect a "checkpoint
completed" line that's not the one we want.  Change the config to try
and prevent this.

Per buildfarm

While at it, update one comment that was forgotten in commit
d18e7566.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20210729.162038.534808353849568395.horikyota.ntt@gmail.com

src/backend/access/transam/xlog.c

@@ -9291,7 +9291,7 @@ CreateCheckPoint(int flags)
 	if (!RecoveryInProgress())
 		TruncateSUBTRANS(GetOldestTransactionIdConsideredRunning());
 
-	/* Real work is done, but log and update stats before releasing lock. */
+	/* Real work is done; log and update stats. */
 	LogCheckpointEnd(false);
 
 	/* Reset the process title */

src/test/recovery/t/019_replslot_limit.pl

@@ -173,11 +173,21 @@ ok( !find_in_log(
 		"requested WAL segment [0-9A-F]+ has already been removed"),
 	'check that required WAL segments are still available');
 
-# Advance WAL again, the slot loses the oldest segment.
+# Create one checkpoint, to improve stability of the next steps
+$node_primary->safe_psql('postgres', "CHECKPOINT;");
+
+# Prevent other checkpoints from occurring while advancing WAL segments
+$node_primary->safe_psql('postgres',
+	"ALTER SYSTEM SET max_wal_size='40MB'; SELECT pg_reload_conf()");
+
+# Advance WAL again. The slot loses the oldest segment by the next checkpoint
 my $logstart = get_log_size($node_primary);
 advance_wal($node_primary, 7);
 
-# wait until the WARNING is issued
+# Now create another checkpoint and wait until the WARNING is issued
+$node_primary->safe_psql('postgres',
+	'ALTER SYSTEM RESET max_wal_size; SELECT pg_reload_conf()');
+$node_primary->safe_psql('postgres', "CHECKPOINT;");
 my $invalidated = 0;
 for (my $i = 0; $i < 10000; $i++)
 {