Fix logical decoding regression tests to correctly check slot existence.

The regression tests for logical decoding verify whether a logical slot
exists or has been dropped. Previously, these tests attempted to
retrieve "slot_name" from the result of slot(), but since "slot_name" was
not included in the result, slot()->{'slot_name'} always returned undef,
leading to incorrect behavior.

This commit fixes the issue by checking the "plugin" field in the result
of slot() instead, ensuring the tests properly verify slot existence.

Back-patch to all supported versions.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/OSCPR01MB149667EC4E738769CA80B7EA5F5AE2@OSCPR01MB14966.jpnprd01.prod.outlook.com
Backpatch-through: 13

src/test/recovery/t/006_logical_decoding.pl

@@ -158,8 +158,8 @@ SKIP:
 	is($node_primary->psql('postgres', 'DROP DATABASE otherdb'),
 		3, 'dropping a DB with active logical slots fails');
 	$pg_recvlogical->kill_kill;
-	is($node_primary->slot('otherdb_slot')->{'slot_name'},
-		undef, 'logical slot still exists');
+	is($node_primary->slot('otherdb_slot')->{'plugin'},
+		'test_decoding', 'logical slot still exists');
 }
 
 $node_primary->poll_query_until('otherdb',
@@ -168,8 +168,8 @@ $node_primary->poll_query_until('otherdb',
 
 is($node_primary->psql('postgres', 'DROP DATABASE otherdb'),
 	0, 'dropping a DB with inactive logical slots succeeds');
-is($node_primary->slot('otherdb_slot')->{'slot_name'},
-	undef, 'logical slot was actually dropped with DB');
+is($node_primary->slot('otherdb_slot')->{'plugin'},
+	'', 'logical slot was actually dropped with DB');
 
 # Test logical slot advancing and its durability.
 my $logical_slot = 'logical_slot';

src/test/recovery/t/010_logical_decoding_timelines.pl

@@ -94,8 +94,8 @@ is( $node_replica->safe_psql(
 		'postgres', q[SELECT 1 FROM pg_database WHERE datname = 'dropme']),
 	'',
 	'dropped DB dropme on standby');
-is($node_primary->slot('dropme_slot')->{'slot_name'},
-	undef, 'logical slot was actually dropped on standby');
+is($node_primary->slot('dropme_slot')->{'plugin'},
+	'', 'logical slot was actually dropped on standby');
 
 # Back to testing failover...
 $node_primary->safe_psql('postgres',