htlcswitch: add new TestUpdateFailMalformedHTLCErrorConversion test

In this commit, we add a new test to ensure that we're able to properly
convert malformed HTLC errors that are sourced from multiple hops away,
or our direct channel peers. In order to test this effectively, we force
the onion decryptors of various peers to always fail which will trigger
the malformed HTLC logic.

htlcswitch/test_utils.go

@@ -589,15 +589,18 @@ func generateRoute(hops ...ForwardingInfo) ([lnwire.OnionPacketSize]byte, error)
 
 // threeHopNetwork is used for managing the created cluster of 3 hops.
 type threeHopNetwork struct {
-	aliceServer      *mockServer
-	aliceChannelLink *channelLink
+	aliceServer       *mockServer
+	aliceChannelLink  *channelLink
+	aliceOnionDecoder *mockIteratorDecoder
 
 	bobServer            *mockServer
 	firstBobChannelLink  *channelLink
 	secondBobChannelLink *channelLink
+	bobOnionDecoder      *mockIteratorDecoder
 
-	carolServer      *mockServer
-	carolChannelLink *channelLink
+	carolServer       *mockServer
+	carolChannelLink  *channelLink
+	carolOnionDecoder *mockIteratorDecoder
 
 	hopNetwork
 }
@@ -948,15 +951,18 @@ func newThreeHopNetwork(t testing.TB, aliceChannel, firstBobChannel,
 	}
 
 	return &threeHopNetwork{
-		aliceServer:      aliceServer,
-		aliceChannelLink: aliceChannelLink.(*channelLink),
+		aliceServer:       aliceServer,
+		aliceChannelLink:  aliceChannelLink.(*channelLink),
+		aliceOnionDecoder: aliceDecoder,
 
 		bobServer:            bobServer,
 		firstBobChannelLink:  firstBobChannelLink.(*channelLink),
 		secondBobChannelLink: secondBobChannelLink.(*channelLink),
+		bobOnionDecoder:      bobDecoder,
 
-		carolServer:      carolServer,
-		carolChannelLink: carolChannelLink.(*channelLink),
+		carolServer:       carolServer,
+		carolChannelLink:  carolChannelLink.(*channelLink),
+		carolOnionDecoder: carolDecoder,
 
 		hopNetwork: *hopNetwork,
 	}