Wait for the monitor in mxs559_block_master

The test seems to fail from time to time due to the monitor not picking up
the master state fast enough. To prevent this, the test needs to wait for
at least one monitor interval.

maxscale-system-test/mxs559_block_master.cpp

@@ -114,6 +114,7 @@ int main(int argc, char* argv[])
     test.try_query(test.maxscales->conn_rwsplit[0], "DROP TABLE IF EXISTS t1");
     test.maxscales->close_maxscale_connections(0);
 
+    test.maxscales->wait_for_monitor();
     test.check_maxscale_alive(0);
     test.log_excludes(0, "due to authentication failure");
     test.log_excludes(0, "due to handshake failure");