htlcswitch/test: more realistic mock encryption

This mock is used in the switch test TestUpdateFailMalformedHTLCErrorConversion.
But because the mock isn't very realistic, it doesn't detect problems
in the handling of malformed failures in the link.

htlcswitch/switch_test.go

@@ -1,7 +1,6 @@
 package htlcswitch
 
 import (
-	"bytes"
 	"crypto/rand"
 	"crypto/sha256"
 	"fmt"
@@ -23,7 +22,6 @@ import (
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/ticker"
-	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
@@ -4088,10 +4086,10 @@ func TestSwitchHoldForward(t *testing.T) {
 	expectedFailure := &lnwire.FailInvalidOnionKey{
 		OnionSHA256: shaOnionBlob,
 	}
-	var b bytes.Buffer
-	require.NoError(t, lnwire.EncodeFailure(&b, expectedFailure, 0))
 
-	assert.Equal(t, lnwire.OpaqueReason(b.Bytes()), failPacket.Reason)
+	fwdErr, err := newMockDeobfuscator().DecryptError(failPacket.Reason)
+	require.NoError(t, err)
+	require.Equal(t, expectedFailure, fwdErr.WireMessage())
 
 	assertNumCircuits(t, c.s, 0, 0)
 
@@ -5515,10 +5513,13 @@ func testSwitchAliasInterceptFail(t *testing.T, zeroConf bool) {
 		failHtlc, ok := failPacket.htlc.(*lnwire.UpdateFailHTLC)
 		require.True(t, ok)
 
-		r := bytes.NewReader(failHtlc.Reason)
-		failure, err := lnwire.DecodeFailure(r, 0)
+		fwdErr, err := newMockDeobfuscator().DecryptError(
+			failHtlc.Reason,
+		)
 		require.NoError(t, err)
 
+		failure := fwdErr.WireMessage()
+
 		failureMsg, ok := failure.(*lnwire.FailTemporaryChannelFailure)
 		require.True(t, ok)