Fix test failure , and add galera_restart_on_unknown_option to disabled.

mysql-test/suite/galera/disabled.def

@@ -55,3 +55,4 @@ galera.galera_ist_restart_joiner : MDEV-12319 #Failed to start mysqld
 galera.galera_ssl_compression : MDEV-12319 #Failed to start mysqld
 galera.galera_wan : MDEV-12319 #Failed to start mysqld
 galera.rpl_row_annotate : MDEV-12319 #Failed to start mysqld
+galera_restart_on_unknown_option : Solve

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

+CALL mtr.add_suppression("WSREP: Refusing exit for the last slave thread.");
 CREATE TABLE t1 (f1 INT PRIMARY KEY) Engine=InnoDB;
 CREATE TABLE t2 (f1 INT AUTO_INCREMENT PRIMARY KEY) Engine=InnoDB;
 SET GLOBAL wsrep_slave_threads = 0;

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

@@ -6,7 +6,7 @@
 
 --source include/galera_cluster.inc
 --source include/have_innodb.inc
-
+CALL mtr.add_suppression("WSREP: Refusing exit for the last slave thread.");
 --let $wsrep_slave_threads_orig = `SELECT @@wsrep_slave_threads`
 
 --connection node_1