Add dedicated test vectors for response token headers

The test suites import test vectors for request and response tokens. The request tokens clearly specify the expected serialisation with `PrivateToken challenge=...,token-key=...`.

This is not the case for response tokens. These are tested based on the expected behaviour that serializing and deserializing a given token is similar to applying the identity, and should result in the same token.
For more robust tests, it would be good to have these as dedicated tests vectors.

Related to #11.

Start by locating the existing request-token test vectors and the tests that serialize and deserialize response tokens. Add dedicated response-token vectors that state the expected header serialization, then verify the response-token tests compare serialization and deserialization against those vectors.

Written by the indexing model from the issue text.