Test justice tx formation from persistence

Test justice tx formation from persistence

Here we implement `WatchtowerPersister`, which provides a test-only
sample implementation of `Persist` similar to how we might imagine a
user to build watchtower-like functionality in the persistence pipeline.

We test that the `WatchtowerPersister` is able to successfully build and
sign a valid justice transaction that sweeps a counterparty's funds if
they broadcast an old commitment.