Add a test for correctness proof generation and checking

[fjarri]

Basically, what do we do when the session returns SessionOutcome::StalledWithProof? The test should include assembling such proofs from all the nodes and verifying them as if it was a third-party adjudicator.

In a realistic scenario, what has to be attached to these proofs? We probably need the same system as with provable errors, where it can provide a list of rounds to save messages from, but the messages would be from this node, not from another node.