Unknown system variable 'debug_dbug' in sst/ist galera tests

CURRENT_TEST: galera.galera_sst_xtrabackup-v2

mysqltest: In included file "./suite/galera/include/galera_st_kill_slave_ddl.inc": 

included from /home/a/e1/m1-10.1/src/mysql-test/suite/galera/t/galera_sst_xtrabackup-v2.test at line 15:

At line 24: query 'let $debug_orig = `SELECT @@debug_dbug`' failed: 1193: Unknown system variable 'debug_dbug'

$ grep -r debug_dbug * | grep -v .result

include/galera_st_kill_slave_ddl.inc:--let $debug_orig = `SELECT @@debug_dbug`

include/galera_st_kill_slave_ddl.inc:SET GLOBAL debug_dbug = 'd,sync.alter_opened_table';

include/galera_st_kill_slave_ddl.inc:SET GLOBAL debug_dbug = $debug_orig;

$ grep -r galera_st_kill_slave_ddl.inc

t/galera_ist_mysqldump.test:--source suite/galera/include/galera_st_kill_slave_ddl.inc

t/galera_ist_xtrabackup-v2.test:--source suite/galera/include/galera_st_kill_slave_ddl.inc

t/galera_sst_rsync.test:--source suite/galera/include/galera_st_kill_slave_ddl.inc

t/galera_sst_mysqldump.test:--source suite/galera/include/galera_st_kill_slave_ddl.inc

t/galera_ist_rsync.test:--source suite/galera/include/galera_st_kill_slave_ddl.inc

t/galera_sst_xtrabackup-v2.test:--source suite/galera/include/galera_st_kill_slave_ddl.inc

t/galera_ist_innodb_flush_logs.test:--source suite/galera/include/galera_st_kill_slave_ddl.inc