Request GTID in mysqlmon_failover_rejoin_old_slave

The test won't otherwise work as the failover is not done without it.

maxscale-system-test/verify_master_failure.cpp

@@ -8,6 +8,7 @@
 
 int main(int argc, char *argv[])
 {
+    Mariadb_nodes::require_gtid(true);
     TestConnections test(argc, argv);
 
     test.tprintf("Blocking master and checking that master failure is delayed at least once.");