fix: ensure that the test slot gets dropped

Based on a quick test, a rollback of the txn does not remove the
logical replication slot that gets created.

Author: darora

migrations/tests/extensions/11-wal2json.sql

@@ -1,4 +1,5 @@
 BEGIN;
 select pg_drop_replication_slot(slot_name) from pg_replication_slots where slot_name = 'test_slot';
 select * from pg_create_logical_replication_slot('test_slot', 'wal2json');
+select pg_drop_replication_slot(slot_name) from pg_replication_slots where slot_name = 'test_slot';
 ROLLBACK;