Add a test for checking `KeyNotInDescriptor` error

[danielabrozzoni]

We return an error if the user gives us a cpfp private key that is not contained in the cpfp descriptor:

https://github.com/revault/revaultd/blob/d4d78e368db8dd4e6b09a1d06013e7036520428e/src/daemon/revaultd.rs#L412

This needs to be unit tested, the right place to do so is probably here:

https://github.com/revault/revaultd/blob/d4d78e368db8dd4e6b09a1d06013e7036520428e/src/daemon/revaultd.rs#L688