Test preimage claim after reorg of counterparty commitment

Test preimage claim after reorg of counterparty commitment

This test adds coverage for receiving a preimage after seeing a
counterparty commitment confirm, followed by a reorg and the
confirmation of a different commitment instead.

The first test covers the case where a holder commitment confirms after
the counterparty commitment reorg.

The second test covers the case where a previous counterparty commitment
confirms after the latest counterparty commitment reorg.