Clean up test handling of resending responding commitment_signed

Clean up test handling of resending responding commitment_signed

When we need to rebroadcast a `commitment_signed` on reconnect in
response to a previous update (ie not one which contains any
updates) we previously hacked in support for it by passing a `-1`
for the number of expected update_add_htlcs. This is a mess, and
with the introduction of `ReconnectArgs` we can now clean it up
easily with a new bool.