Finality violation test coverage

[arhag]

Add a finality violation verification contract.

Create test that forces a finality violation.

In the test, have something acting like a light client that verifies finality proofs in the false chain and stores minimum required information to be able to prove future possible finality violations. Then discovers to real chain and uses data from that chain and its locally stored information from the false chain to construct a valid finality violation proof that is verified by the contract.