Code review comment for lp:~afrantzis/mir/fix-1445418-release-surfaces

OK as a workaround but slightly changes the test. It would be more consistent to close the surfaces at the end of the test (and not in the test loop). Vis:

        received.wait_for(400ms);
    }

    for (auto const surface : client_surface)
        mir_surface_release_sync(surface);
}