fixed set test scene

[huguesdevimeux]

Hot fix for set_test_scene, which was broken by #532.

We need a test for this, that's why it's WIP.

close #571

[huguesdevimeux]

In fact I'm not sure how to test it and whether it should be so.
Because testing this is basically testing scene.render, and all the rendering process.

[behackl]

LGTM.

[kolibril13]

I don't understand these lines

 assert not np.all(
        data == np.array([0, 0, 0, 255])
    ), f"Control data generated for {str(scene)} only contains empty pixels."
    assert data.shape == (480, 854, 4)

completely.
However, I think it is important to merge this as soon as possible, as other prs really depend on this one (including the new one #580), and then we can come back to discuss how we want to test this in a future pr.