lnwallet: return instead evaluated commitment instead of mutating

createCommitmentTx would earlier mutate the passed commitment struct
after evaluating the htlc view and calculating the final balances, which
was confusing since the balances are supposed to only be *after*
subtracting fees.

Instead we take the needed parameters as arguments, and return the final
balances, tx and fee to populate the commitment struct in a proper way.

lnwallet/transactions_test.go

@@ -794,23 +794,32 @@ func TestCommitmentAndHTLCTransactions(t *testing.T) {
 		}
 		theHTLCView := htlcViewFromHTLCs(htlcs)
 
+		feePerKw := chainfee.SatPerKWeight(test.commitment.FeePerKw)
+		isOurs := true
+		height := test.commitment.CommitHeight
+
 		// Create unsigned commitment transaction.
-		commitmentView := &commitment{
-			height:       test.commitment.CommitHeight,
-			ourBalance:   test.commitment.LocalBalance,
-			theirBalance: test.commitment.RemoteBalance,
-			feePerKw:     chainfee.SatPerKWeight(test.commitment.FeePerKw),
-			dustLimit:    tc.dustLimit,
-			isOurs:       true,
-		}
-		err = channel.commitBuilder.createCommitmentTx(
-			commitmentView, theHTLCView, keys,
+		view, err := channel.commitBuilder.createUnsignedCommitmentTx(
+			test.commitment.LocalBalance,
+			test.commitment.RemoteBalance, isOurs, feePerKw,
+			height, theHTLCView, keys,
 		)
 		if err != nil {
 			t.Errorf("Case %d: Failed to create new commitment tx: %v", i, err)
 			continue
 		}
 
+		commitmentView := &commitment{
+			ourBalance:   view.ourBalance,
+			theirBalance: view.theirBalance,
+			txn:          view.txn,
+			fee:          view.fee,
+			height:       height,
+			feePerKw:     feePerKw,
+			dustLimit:    tc.dustLimit,
+			isOurs:       isOurs,
+		}
+
 		// Initialize LocalCommit, which is used in getSignedCommitTx.
 		channelState.LocalCommitment = test.commitment
 		channelState.LocalCommitment.Htlcs = htlcs