Expose revokeable output index and building a justice tx from commitment

Expose revokeable output index and building a justice tx from commitment

For watchtowers to be able to build justice transactions for our
counterparty's revoked commitments, they need to be able to find the
revokeable output for them to sweep. Here we cache `to_self_delay` in
`CommitmentTransaction` to allow for finding this output on the struct
directly. We also add a simple helper method to aid in building the
initial spending transaction.

This also adds a unit test for both of these helpers, and
refactors a bit of a previous `CommitmentTransaction` unit test to make
adding these easier.