MDEV-13873: galera.galera_suspend_slave failed in buildbot with wrong error code

Add wsrep_sync_wait as we want INSERT to fail.

mysql-test/suite/galera/r/galera_suspend_slave.result

 CREATE TABLE t1 (f1 INTEGER PRIMARY KEY) ENGINE=InnoDB;
 Suspending node_2 ...
 SET SESSION wsrep_sync_wait = 0;
+SET SESSION wsrep_sync_wait = 15;
 INSERT INTO t1 VALUES (1);
 Got one of the listed errors
 Resuming node_2 ...
+SET SESSION wsrep_sync_wait = 0;
 INSERT INTO t1 VALUES (1);
 SET SESSION wsrep_sync_wait = 0;
 SELECT COUNT(*) = 1 FROM t1;

mysql-test/suite/galera/t/galera_suspend_slave.test

@@ -34,6 +34,8 @@ SET SESSION wsrep_sync_wait = 0;
 --let $wait_condition = SELECT VARIABLE_VALUE = 1 FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME = 'wsrep_cluster_size';
 --source include/wait_condition.inc
 
+SET SESSION wsrep_sync_wait = 15;
+
 --error ER_UNKNOWN_COM_ERROR,ER_LOCK_WAIT_TIMEOUT,ER_LOCK_DEADLOCK,ER_ERROR_DURING_COMMIT
 INSERT INTO t1 VALUES (1);
 
@@ -46,6 +48,7 @@ INSERT INTO t1 VALUES (1);
 	exit(0);
 EOF
 
+SET SESSION wsrep_sync_wait = 0;
 --source include/wait_until_ready.inc
 INSERT INTO t1 VALUES (1);