Fix BF failure in commit 93db6cbda0.

The code to match the required LOG in the test was not robust enough to
match it. It was using a very specific format to search the required
message which doesn't work when one uses log_error_verbosity = verbose.

Author: Hou Zhijie
Discussion: https://postgr.es/m/CAA4eK1KcQSk7wzC7Zfrth9OhrjW2HvxL4tKgU42qqH7p6jn+FA@mail.gmail.com

src/test/recovery/t/040_standby_failover_slots_sync.pl

@@ -373,7 +373,7 @@ $standby1->append_conf('postgresql.conf', qq(sync_replication_slots = on));
 $standby1->reload;
 
 # Confirm that the slot sync worker is able to start.
-$standby1->wait_for_log(qr/LOG:  slot sync worker started/,
+$standby1->wait_for_log(qr/slot sync worker started/,
 	$log_offset);
 
 $log_offset = -s $standby1->logfile;
@@ -384,9 +384,9 @@ $standby1->reload;
 
 # Confirm that slot sync worker acknowledge the GUC change and logs the msg
 # about wrong configuration.
-$standby1->wait_for_log(qr/LOG:  slot sync worker will restart because of a parameter change/,
+$standby1->wait_for_log(qr/slot sync worker will restart because of a parameter change/,
 	$log_offset);
-$standby1->wait_for_log(qr/LOG:  slot synchronization requires hot_standby_feedback to be enabled/,
+$standby1->wait_for_log(qr/slot synchronization requires hot_standby_feedback to be enabled/,
 	$log_offset);
 
 $log_offset = -s $standby1->logfile;
@@ -396,7 +396,7 @@ $standby1->append_conf('postgresql.conf', "hot_standby_feedback = on");
 $standby1->reload;
 
 # Confirm that the slot sync worker is able to start now.
-$standby1->wait_for_log(qr/LOG:  slot sync worker started/,
+$standby1->wait_for_log(qr/slot sync worker started/,
 	$log_offset);
 
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