Change the LOG level in 040_standby_failover_slots_sync.pl to DEBUG2.

Temporarily change the log level of 040_standby_failover_slots_sync.pl to
DEBUG2. This is to get more information about BF failures. We will reset
it back to default once the tests are stabilized.

Author: Hou Zhijie
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/514f6f2f-6833-4539-39f1-96cd1e011f23@enterprisedb.com
Discussion: https://postgr.es/m/OS0PR01MB571633C23B2A4CAC5FB0371A944C2@OS0PR01MB5716.jpnprd01.prod.outlook.com

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

@@ -130,14 +130,20 @@ $standby1->init_from_backup(
 	has_streaming => 1,
 	has_restoring => 1);
 
+# Increase the log_min_messages setting to DEBUG2 on both the standby and
+# primary to debug test failures, if any.
 my $connstr_1 = $primary->connstr;
 $standby1->append_conf(
 	'postgresql.conf', qq(
 hot_standby_feedback = on
 primary_slot_name = 'sb1_slot'
 primary_conninfo = '$connstr_1 dbname=postgres'
+log_min_messages = 'debug2'
 ));
 
+$primary->append_conf('postgresql.conf', "log_min_messages = 'debug2'");
+$primary->reload;
+
 $primary->psql('postgres',
 	q{SELECT pg_create_logical_replication_slot('lsub2_slot', 'test_decoding', false, false, true);}
 );
@@ -265,6 +271,13 @@ is( $standby1->safe_psql(
 	"t",
 	'logical slot is re-synced');
 
+# Reset the log_min_messages to the default value.
+$primary->append_conf('postgresql.conf', "log_min_messages = 'warning'");
+$primary->reload;
+
+$standby1->append_conf('postgresql.conf', "log_min_messages = 'warning'");
+$standby1->reload;
+
 ##################################################
 # Test that a synchronized slot can not be decoded, altered or dropped by the
 # user