Fix quadtree tests that check the children of a node

Currently, the quadtree tests try to check the children of a node like so:

    assert.isUndefined(node[2]);

However, node is the result of d3_geom_quadtreeNode, not an array, so that assert is just checking the '2' property of the node object, which, indeed happens to be undefined.

I think these tests mean to check the `nodes` property of the node, which *is* an array, so this updates those statements like so:

    assert.isUndefined(node.nodes[2]);