lnd test: fix csv mempool sweep time
lnd test: fix csv mempool sweep time

Since CSV locked outputs specifies the first block where they are
allowed to be included, they can actually be added one block earlier
into the mempool.

This led to a flake, where the sweep tx was already in the mempool at
the time we mined the last block, causing the next mempool check to
fail.

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