I think that might be a cool thing to be added if the game ever does get updated - probably it will never be thrown in, unless another few patches come out first(; Thanks for correctin' me, anyways<3
I think the reason it wasn't added is because it's not exactly necessary.
I use the test lion just to look at object placement, size comparison, collisions, etc., so it doesn't need to really be customizable.
Though, if at any point you want to test a map using your character, it's really easy to do so. Every time you save the map, it updates a .map file for it. So all you have to do is log in, have a character climb through the portal, and leave it there, so whenever you log in, it's in that map, and you can look at your new stuff.
Yet I will say, it would be nicer to just be able to customize that test lion. xD