itest/channel_force_close test: handle HTLCs going through sweeper
itest/channel_force_close test: handle HTLCs going through sweeper

Now that the HTLC second-level transactions are going through the
sweeper instead of the nursery, there are a few things we must account
for.
1. The sweeper sweeps the CSV locked HTLC output one block earlier than
   the nursery.
2. The sweeper aggregates several HTLC second levels into one
   transaction. This also means it is not enough to check txids of the
   transactions spent by the final sweep, but we must use the actual
   outpoint to distinguish.

aakselrod/lnd,aakselrod/lnd