htlcswitch: update unit tests to account for recent API changes

htlcswitch/mock.go

@@ -604,6 +604,10 @@ func (f *mockChannelLink) HandleChannelUpdate(lnwire.Message) {
 
 func (f *mockChannelLink) UpdateForwardingPolicy(_ ForwardingPolicy) {
 }
+func (f *mockChannelLink) HtlcSatifiesPolicy([32]byte, lnwire.MilliSatoshi,
+	lnwire.MilliSatoshi) lnwire.FailureMessage {
+	return nil
+}
 
 func (f *mockChannelLink) Stats() (uint64, lnwire.MilliSatoshi, lnwire.MilliSatoshi) {
 	return 0, 0, 0