This could use a comment about what it is doing at a high level.
Please double check why one of the tests is failing.
Did you use some other tests as reference here? How to decide what goes into NodeTreeTestContext and what into NodeTreeTest?
Better change ntreeIsRegistered to take a const node tree.
Comment style, missing dot. Same in other places.
The name here should contain sq for squared.
ConstListbaseWrapper for iterating const ListBase *
While this works, I don't always find this better than the macro version. Mainly because it is more verbose and don't add much of a benefit (it doesn't add type safety in this case). I could see this being useful if you want to pass a typed listbase to another function though.
ConstListbaseWrapper for iterating const ListBase *
Try if using using ListBaseWrapperTemplate<ListBase, T>::ListBaseWrapperTemplate; instead of implementing the constructors again. (not 100% sure if this works here, if it doesn't, ignore this comment)