routing/pathfind_test: allow custom node features
routing/pathfind_test: allow custom node features

This commit allows custom node features to be populated in specific test
instances. For consistency, we auto-populate an empty feature vector for
nodes that have nil feature vectors before writing them to the database.

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