Force synchronous commit in new-ish test_decoding test.

This was missed in a924c327 ff.

contrib/test_decoding/expected/slot.out

@@ -1,3 +1,5 @@
+-- predictability
+SET synchronous_commit = on;
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_p', 'test_decoding');
  ?column? 
 ----------
@@ -31,6 +33,7 @@ SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_t2', 'tes
 -- here we want to start a new session and wait till old one is gone
 select pg_backend_pid() as oldpid \gset
 \c -
+SET synchronous_commit = on;
 do 'declare c int = 0;
 begin
   while (select count(*) from pg_replication_slots where active_pid = '

contrib/test_decoding/sql/slot.sql

@@ -1,3 +1,6 @@
+-- predictability
+SET synchronous_commit = on;
+
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_p', 'test_decoding');
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_t', 'test_decoding', true);
 
@@ -9,6 +12,8 @@ SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_t2', 'tes
 -- here we want to start a new session and wait till old one is gone
 select pg_backend_pid() as oldpid \gset
 \c -
+SET synchronous_commit = on;
+
 do 'declare c int = 0;
 begin
   while (select count(*) from pg_replication_slots where active_pid = '