lnwallet/test: make use of test htlcs explicit

Previously whether or not to add test htlcs was implictly controlled by
a nil value of the HtlcDescs test parameter. With the conversion to
json, that nil value got lost.

The reason that the test still passed is because with the fee rate of
the no-htlc test case, the htlcs were trimmed. Also because in the test
json, balances are specified after applying htlcs, the test didn't fail
with a mismatching balance.

lnwallet/transactions_test.go

@@ -139,10 +139,15 @@ type htlcDesc struct {
 }
 
 type testCase struct {
-	Name                    string
-	LocalBalance            lnwire.MilliSatoshi
-	RemoteBalance           lnwire.MilliSatoshi
-	FeePerKw                btcutil.Amount
+	Name          string
+	LocalBalance  lnwire.MilliSatoshi
+	RemoteBalance lnwire.MilliSatoshi
+	FeePerKw      btcutil.Amount
+
+	// UseTestHtlcs defined whether the fixed set of test htlc should be
+	// added to the channel before checking the commitment assertions.
+	UseTestHtlcs bool
+
 	HtlcDescs               []htlcDesc
 	ExpectedCommitmentTxHex string
 	RemoteSigHex            string
@@ -252,7 +257,7 @@ func testVectors(t *testing.T, chanType channeldb.ChannelType, test testCase) {
 	remoteBalance := test.RemoteBalance
 	localBalance := test.LocalBalance
 
-	if test.HtlcDescs != nil {
+	if test.UseTestHtlcs {
 		for _, htlc := range testHtlcs {
 			if htlc.incoming {
 				remoteBalance += htlc.amount
@@ -276,7 +281,7 @@ func testVectors(t *testing.T, chanType channeldb.ChannelType, test testCase) {
 	// map that allows us to identify the htlcs in the scripts later on and
 	// retrieve the corresponding preimage.
 	var hash160map map[[20]byte]lntypes.Preimage
-	if test.HtlcDescs != nil {
+	if test.UseTestHtlcs {
 		hash160map = addTestHtlcs(t, remoteChannel, localChannel)
 	}