Sync slaves before checking MaxScale is alive

Synchronizing the slaves before checking that MaxScale is still alive
makes sure the slave servers have settled down to a known state.

maxscale-system-test/different_size_rwsplit.cpp

@@ -19,6 +19,7 @@ int main(int argc, char *argv[])
 
     different_packet_size(Test, false);
 
+    Test->repl->sync_slaves();
     Test->check_maxscale_alive(0);
     int rval = Test->global_result;
     delete Test;