lnwallet: ensure that each backend uses a distinct seed in integration tests

In this commit, we add an additional degree of isolation to the set of
integration tests. A bug was recently fixed to ensure that the wallet
always starts rescans from _after_ it's birthday. In the past it would
miss some funds that were deposited _right_ before the birthday of the
wallet. Fixing this bug exposed a test flake wherein the btcd node would
itself rescan back and collect some of the funds that were last sent to
the bitcoind node.

In order to fix this, we now ensure that each backend will use a unique
HD seed such that the tests are still deterministic for each backend and
role.

lnwallet/interface_test.go

@@ -2,6 +2,7 @@ package lnwallet_test
 
 import (
 	"bytes"
+	"crypto/sha256"
 	"encoding/hex"
 	"fmt"
 	"io/ioutil"
@@ -2104,6 +2105,7 @@ func runTests(t *testing.T, walletDriver *lnwallet.WalletDriver,
 	backEnd string, miningNode *rpctest.Harness,
 	rpcConfig rpcclient.ConnConfig,
 	chainNotifier *btcdnotify.BtcdNotifier) {
+
 	var (
 		bio lnwallet.BlockChainIO
 
@@ -2282,9 +2284,14 @@ func runTests(t *testing.T, walletDriver *lnwallet.WalletDriver,
 			t.Fatalf("unknown chain driver: %v", backEnd)
 		}
 
+		aliceSeed := sha256.New()
+		aliceSeed.Write([]byte(backEnd))
+		aliceSeed.Write(aliceHDSeed[:])
+		aliceSeedBytes := aliceSeed.Sum(nil)
+
 		aliceWalletConfig := &btcwallet.Config{
 			PrivatePass:  []byte("alice-pass"),
-			HdSeed:       aliceHDSeed[:],
+			HdSeed:       aliceSeedBytes,
 			DataDir:      tempTestDirAlice,
 			NetParams:    netParams,
 			ChainSource:  aliceClient,
@@ -2301,9 +2308,14 @@ func runTests(t *testing.T, walletDriver *lnwallet.WalletDriver,
 			keychain.CoinTypeTestnet,
 		)
 
+		bobSeed := sha256.New()
+		bobSeed.Write([]byte(backEnd))
+		bobSeed.Write(bobHDSeed[:])
+		bobSeedBytes := bobSeed.Sum(nil)
+
 		bobWalletConfig := &btcwallet.Config{
 			PrivatePass:  []byte("bob-pass"),
-			HdSeed:       bobHDSeed[:],
+			HdSeed:       bobSeedBytes,
 			DataDir:      tempTestDirBob,
 			NetParams:    netParams,
 			ChainSource:  bobClient,