sporadic failures of rpl.rpl_parallel_multi_domain_xa

it's a slow test, the slave needs to catch up, reading >1500
transactions. A default MASTER_GTID_WAIT() timeout in
sync_with_master_gtid.inc is 120 seconds, which might be not
enough for a slow/overloaded slave.

Let's wait forever or until ./mtr --testcase-timeout,
whatever comes first.

mysql-test/suite/rpl/t/rpl_parallel_multi_domain_xa.test

@@ -9,6 +9,7 @@
 
 # Tests' global declarations
 --let $trx = _trx_
+--let $slave_timeout= -1
 
 call mtr.add_suppression("Deadlock found when trying to get lock; try restarting transaction");
 call mtr.add_suppression("WSREP: handlerton rollback failed");