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:

revaultd/src/daemon/revaultd.rs

Line 412 in d4d78e3

.ok_or(CpfpKeyError::KeyNotInDescriptor(pubkey.to_string()))?;

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

revaultd/src/daemon/revaultd.rs

Line 688 in d4d78e3

fn test_from_config() {