In this commit, we update the channel state machine tests to use a new
key for each purpose. Before this commit, the same key would be used
the entire time. As a result, a few bugs slipped by that would’ve been
detected if we used fresh keys for each purpose. Additionally, this
reflect the real world case as we always use distinct keys for each
purpose to avoid key re-use.
To implement the BOLT 03 test vectors, a more powerful mockSigner is
required. The new version of mockSigner stores multiple keys and signs
the transaction outputs with the appropriate one.
