channeldb/graph_test: extend prune edge update index test to update edges
channeldb/graph_test: extend prune edge update index test to update edges

In this commit, we extend TestChannelEdgePruningUpdateIndexDeletion test
to include one more update for each edge. By doing this, we can
correctly determine whether old entries were properly pruned from the
index once a new update has arrived.

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