routing: fix node ordering in test graph

There is the general assumption that channel edge policy nodes are
ordered such that the node1 pubkey is smaller than the key of node 2. In
the test graph, this assumption didn't hold. This commit fixes the test
graph and also adds a check to prevent this from happening again.

routing/pathfind_test.go

@@ -7,6 +7,7 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io/ioutil"
 	"math"
 	"math/big"
@@ -232,6 +233,13 @@ func parseTestGraph(path string) (*testGraphInstance, error) {
 			return nil, err
 		}
 
+		if bytes.Compare(node1Bytes, node2Bytes) == 1 {
+			return nil, fmt.Errorf(
+				"channel %v node order incorrect",
+				edge.ChannelID,
+			)
+		}
+
 		fundingTXID := strings.Split(edge.ChannelPoint, ":")[0]
 		txidBytes, err := chainhash.NewHashFromStr(fundingTXID)
 		if err != nil {