lntest+routing: update best height after graph pruning

It seems #5246 introduced a subtle bug that lead to the error "out of
order block: expecting height=1, got height=XXX" some times during
startup. Apparently it can happen that during pruning of the graph tip
some blocks can come in before we start our chain view and the new block
subscription. By querying the chain backend for the best height before
syncing with the graph we ensure that we never miss a block.

routing/router.go

@@ -563,6 +563,14 @@ func (r *ChannelRouter) Start() error {
 			}
 		}
 
+		// The graph pruning might have taken a while and there could be
+		// new blocks available.
+		bestHash, bestHeight, err = r.cfg.Chain.GetBestBlock()
+		if err != nil {
+			return err
+		}
+		r.bestHeight = uint32(bestHeight)
+
 		// Before we begin normal operation of the router, we first need
 		// to synchronize the channel graph to the latest state of the
 		// UTXO set.