hild gets `spacing` on its left and right side.
          assert.isTrue(positionEquals(c1, 0, 50, spacing, spacing + 50), "child 1 position ok");
          var end = spacing + 50 + spacing;
          assert.isTrue(positionEquals(c2, 0, 50, end + spacing, end + spacing + 50), "child 2 position ok");
          end = end + spacing + 50 + spacing;
          assert.isTrue(positionEquals(c3, 0, 50, end + spacing, end + spacing + 50), "child 3 position ok");
        });
      });

      suite('align-content', func