add gosop v3 to the test suite

on github, Lukas Burkhalter wrote:

The gosop implementation on the main branch currently does not support inline signatures (except clearsigned). Nevertheless, an updated version 3 of GopenPGP is currently in development. The gosop-gopenpgp-v3 branch in this repo implements gosop with the new GopenPGP version. It implements the latest api in draft-dkg-openpgp-stateless-cli-06 and implements all the crypto-refresh features as well. Feel free to try this version ;)

It would be great to see this version in the interop test suite.