Confidential Descriptors lack multi-key test cases

Two related issues in https://github.com/ElementsProject/ELIPs/blob/main/elip-0150.mediawiki:

Firstly , all examples/test cases use single (non-blinding) key expressions - i.e. no there are index-derivable keys such as `KEY/*`, `KEY/<0;1>/*` etc.

These are likely the most common use of such descriptors, with single key use-cases limited to sharing single addresses for counterparty address unblinding while multi-key expressions are what wallets will register and use in practice. It would be helpful to have a few such test cases derived at some index `n` for verifying implementations.

Secondly, it seems obvious from the way non-slip77 blinding keys are derived that the blinding key itself (where given as a `KEY` expression) must not contain a path expression. However this isn't explicit and there are no negative test cases where a blinding key contains a path.