Make cube test asymmetric

zipmask_test.m

@@ -7,7 +7,7 @@ zs = zeros(d, d, d);
     function test_cube()
         mask = zs;
         l = ceil(d/4);
-        mask(1:l, 1:l, 1:l) = 1;
+        mask(1:l, 2:l+1, 3:l+1) = 1;
         s = zipmask_pack(m, mask);
         [um, umask] = zipmask_unpack(s);
         assert(isequal(umask, mask));