channel: properly roll over fee from commit tx during co-op chan close

In this commit, we fix an existing bug within our cooperative channel
closing transaction generation. Before this commit, we wouldn’t account
for the fee already allocated within the commitment transaction. As a
result, we would calculate the evaluated balance considering the fee
incorrectly. In this commit, we fix this by adding the commitment fee
to the balance of the initiator when crafting the closing transaction

lnwallet/channel_test.go

@@ -1807,7 +1807,7 @@ func TestCooperativeCloseDustAdherence(t *testing.T) {
 	// Both sides currently have over 1 BTC settled as part of their
 	// balances. As a result, performing a cooperative closure now result
 	// in both sides having an output within the closure transaction.
-	aliceFee := btcutil.Amount(aliceChannel.CalcFee(aliceFeeRate))
+	aliceFee := btcutil.Amount(aliceChannel.CalcFee(aliceFeeRate)) + 1000
 	aliceSig, err := aliceChannel.CreateCloseProposal(aliceFee,
 		aliceDeliveryScript, bobDeliveryScript)
 	if err != nil {
@@ -1815,7 +1815,7 @@ func TestCooperativeCloseDustAdherence(t *testing.T) {
 	}
 	aliceCloseSig := append(aliceSig, byte(txscript.SigHashAll))
 
-	bobFee := btcutil.Amount(bobChannel.CalcFee(bobFeeRate))
+	bobFee := btcutil.Amount(bobChannel.CalcFee(bobFeeRate)) + 1000
 	bobSig, err := bobChannel.CreateCloseProposal(bobFee,
 		bobDeliveryScript, aliceDeliveryScript)
 	if err != nil {
@@ -1874,10 +1874,12 @@ func TestCooperativeCloseDustAdherence(t *testing.T) {
 		t.Fatalf("close tx has wrong number of outputs: expected %v "+
 			"got %v", 1, len(closeTx.TxOut))
 	}
-	if closeTx.TxOut[0].Value != int64(aliceBal.ToSatoshis()-calcStaticFee(0)) {
+	commitFee := aliceChannel.channelState.LocalCommitment.CommitFee
+	aliceExpectedBalance := aliceBal.ToSatoshis() - aliceFee + commitFee
+	if closeTx.TxOut[0].Value != int64(aliceExpectedBalance) {
 		t.Fatalf("alice's balance is incorrect: expected %v, got %v",
-			int64(aliceBal.ToSatoshis()-calcStaticFee(0)),
-			closeTx.TxOut[0].Value)
+			aliceExpectedBalance,
+			int64(closeTx.TxOut[0].Value))
 	}
 
 	// Finally, we'll modify the current balances and dust limits such that