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/mock.go

@@ -368,7 +368,10 @@ func (o *mockObfuscator) EncryptFirstHop(failure lnwire.FailureMessage) (
 
 func (o *mockObfuscator) IntermediateEncrypt(reason lnwire.OpaqueReason) lnwire.OpaqueReason {
 	return reason
+}
 
+func (o *mockObfuscator) EncryptMalformedError(reason lnwire.OpaqueReason) lnwire.OpaqueReason {
+	return reason
 }
 
 // mockDeobfuscator mock implementation of the failure deobfuscator which
@@ -400,6 +403,8 @@ type mockIteratorDecoder struct {
 	mu sync.RWMutex
 
 	responses map[[32]byte][]DecodeHopIteratorResponse
+
+	decodeFail bool
 }
 
 func newMockIteratorDecoder() *mockIteratorDecoder {
@@ -451,6 +456,10 @@ func (p *mockIteratorDecoder) DecodeHopIterators(id []byte,
 			req.OnionReader, req.RHash, req.IncomingCltv,
 		)
 
+		if p.decodeFail {
+			failcode = lnwire.CodeTemporaryChannelFailure
+		}
+
 		resp := DecodeHopIteratorResponse{
 			HopIterator: iterator,
 			FailCode:    failcode,