lntest/itest: add change address recovery case to onchain recovery test

This is mainly motivated by a now fixed bug in the wallet in which
change addresses could at times be created outside of the default key
scopes. Recovery only used to be performed on the default key scopes, so
ideally this test case would've caught the bug earlier.

lntest/itest/lnd_test.go

@@ -799,7 +799,7 @@ func testOnchainFundRecovery(net *lntest.NetworkHarness, t *harnessTest) {
 	// method takes the expected value of Carol's balance when using the
 	// given recovery window. Additionally, the caller can specify an action
 	// to perform on the restored node before the node is shutdown.
-	restoreCheckBalance := func(expAmount int64, expectedNumUTXOs int,
+	restoreCheckBalance := func(expAmount int64, expectedNumUTXOs uint32,
 		recoveryWindow int32, fn func(*lntest.HarnessNode)) {
 
 		// Restore Carol, passing in the password, mnemonic, and
@@ -825,13 +825,7 @@ func testOnchainFundRecovery(net *lntest.NetworkHarness, t *harnessTest) {
 				t.Fatalf("unable to query wallet balance: %v",
 					err)
 			}
-
-			// Verify that Carol's balance matches our expected
-			// amount.
 			currBalance = resp.ConfirmedBalance
-			if expAmount != currBalance {
-				return false
-			}
 
 			utxoReq := &lnrpc.ListUnspentRequest{
 				MaxConfs: math.MaxInt32,
@@ -841,8 +835,13 @@ func testOnchainFundRecovery(net *lntest.NetworkHarness, t *harnessTest) {
 			if err != nil {
 				t.Fatalf("unable to query utxos: %v", err)
 			}
+			currNumUTXOs = uint32(len(utxoResp.Utxos))
 
-			currNumUTXOs := len(utxoResp.Utxos)
+			// Verify that Carol's balance and number of UTXOs
+			// matches what's expected.
+			if expAmount != currBalance {
+				return false
+			}
 			if currNumUTXOs != expectedNumUTXOs {
 				return false
 			}
@@ -958,7 +957,50 @@ func testOnchainFundRecovery(net *lntest.NetworkHarness, t *harnessTest) {
 
 	// Ensure that using a recovery window of 20 succeeds with all UTXOs
 	// found and the final balance reflected.
-	restoreCheckBalance(6*btcutil.SatoshiPerBitcoin, 6, 20, nil)
+
+	// After these checks are done, we'll want to make sure we can also
+	// recover change address outputs.  This is mainly motivated by a now
+	// fixed bug in the wallet in which change addresses could at times be
+	// created outside of the default key scopes. Recovery only used to be
+	// performed on the default key scopes, so ideally this test case
+	// would've caught the bug earlier. Carol has received 6 BTC so far from
+	// the miner, we'll send 5 back to ensure all of her UTXOs get spent to
+	// avoid fee discrepancies and a change output is formed.
+	const minerAmt = 5 * btcutil.SatoshiPerBitcoin
+	const finalBalance = 6 * btcutil.SatoshiPerBitcoin
+	promptChangeAddr := func(node *lntest.HarnessNode) {
+		minerAddr, err := net.Miner.NewAddress()
+		if err != nil {
+			t.Fatalf("unable to create new miner address: %v", err)
+		}
+		ctxt, _ := context.WithTimeout(ctxb, defaultTimeout)
+		resp, err := node.SendCoins(ctxt, &lnrpc.SendCoinsRequest{
+			Addr:   minerAddr.String(),
+			Amount: minerAmt,
+		})
+		if err != nil {
+			t.Fatalf("unable to send coins to miner: %v", err)
+		}
+		txid, err := waitForTxInMempool(
+			net.Miner.Node, minerMempoolTimeout,
+		)
+		if err != nil {
+			t.Fatalf("transaction not found in mempool: %v", err)
+		}
+		if resp.Txid != txid.String() {
+			t.Fatalf("txid mismatch: %v vs %v", resp.Txid,
+				txid.String())
+		}
+		block := mineBlocks(t, net, 1, 1)[0]
+		assertTxInBlock(t, block, txid)
+	}
+	restoreCheckBalance(finalBalance, 6, 20, promptChangeAddr)
+
+	// We should expect a static fee of 27750 satoshis for spending 6 inputs
+	// (3 P2WPKH, 3 NP2WPKH) to two P2WPKH outputs. Carol should therefore
+	// only have one UTXO present (the change output) of 6 - 5 - fee BTC.
+	const fee = 27750
+	restoreCheckBalance(finalBalance-minerAmt-fee, 1, 21, nil)
 }
 
 // commitType is a simple enum used to run though the basic funding flow with