k3/k4.pid vectors should cover PID operations against a signing key

[bannable]

The public component of a signing key will always be used for verifying signed messages, and PASERK users will obviously want to indicate the PID associated with a signed message so that the verifier does not require access to the secret material.

To support this use case, the test vectors should clearly define the expected PID outputs when provided a keypair as well as the current case of only a public key.