Now that we always generate an extra proper VerificationKey for each
Tweak::ChangePubkey case, this /should/ never fail: it also helps split out the
actual verification of the signature from the parsing and validation of the key
itself.
Instead of creating an invalid verification key for a particular signature by tweaking its bytes,
create another verification key and when the ChangePubkey tweak is applied, just swap out the correct
SignatureCase::pk_bytes for SignatureCase::invalid_pk_bytes and check that trying to verify the signature
using that wrong key fails, as expected.
Resolves#2170
Deirdre Connolly
Deirdre Connolly
teor