test: modify sphinx replay test to force close the final channel
test: modify sphinx replay test to force close the final channel

In this commit, we modify the sphinx replay test to actually force close
that final channel. We do this, as otherwise, we'll now reject the co-op
close attempt as the channel still has active HTLCs.

LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd