lntest/itest: wait for on-chain balance restore

We add a wait predicate to make sure the node's on-chain balance is
restored before continuing the restore test case.

This is needed since the DLP test scenario includes several restarts of
the node, and if the node isn't done scanning for on-chain balance
before the restart happens, it would be unlocked without a recovery
window, causing funds to be left undiscovered.

lntest/itest/lnd_test.go

@@ -8121,7 +8121,9 @@ func assertDLPExecuted(net *lntest.NetworkHarness, t *harnessTest,
 	assertTxInBlock(t, block, forceClose)
 
 	// Dave should sweep his funds immediately, as they are not timelocked.
-	daveSweep, err := waitForTxInMempool(net.Miner.Node, minerMempoolTimeout)
+	daveSweep, err := waitForTxInMempool(
+		net.Miner.Node, minerMempoolTimeout,
+	)
 	if err != nil {
 		t.Fatalf("unable to find Dave's sweep tx in mempool: %v", err)
 	}
@@ -13559,6 +13561,27 @@ func testChanRestoreScenario(t *harnessTest, net *lntest.NetworkHarness,
 		t.Fatalf("unable to restore node: %v", err)
 	}
 
+	// First ensure that the on-chain balance is restored.
+	err = wait.NoError(func() error {
+		ctxt, _ := context.WithTimeout(ctxb, defaultTimeout)
+		balReq := &lnrpc.WalletBalanceRequest{}
+		daveBalResp, err := dave.WalletBalance(ctxt, balReq)
+		if err != nil {
+			return err
+		}
+
+		daveBal := daveBalResp.ConfirmedBalance
+		if daveBal <= 0 {
+			return fmt.Errorf("expected positive balance, had %v",
+				daveBal)
+		}
+
+		return nil
+	}, defaultTimeout)
+	if err != nil {
+		t.Fatalf("On-chain balance not restored: %v", err)
+	}
+
 	// Now that we have our new node up, we expect that it'll re-connect to
 	// Carol automatically based on the restored backup.
 	ctxt, _ = context.WithTimeout(ctxb, defaultTimeout)