Tree Builder: Exercise requires sorting of results, but this is not mentioned in the description

Hi, I was send here after [creating a MR in the go repository](https://github.com/exercism/go/pull/2215#issuecomment-1125439298).

Currently the tests of the tree builder exercise assume an order of a node's children, however the description does not specify anything about the order.

Example:
Build for test case "three nodes in order" returned 0:[2:[] 1:[]] but was expected to return 0:[1:[] 2:[]].

I think this could be solved in two ways:
- Adding the requirement of sorting the results to the description
- Make the tests agnostic to the order (for example by sorting actual and expected nodes in the tests before comparing them). However (afaik) this would require adapting the tests in each track, eg what I've done in the linked MR for go.

I'm happy to expand the description if this is the preferred solution, just let me know.