itest: use wait predicate for balance assertion in assertDLPExecuted

This assertion would at times fail if the wallet balance hadn't been
updated yet.

lntest/itest/lnd_test.go

@@ -9705,18 +9705,26 @@ func assertDLPExecuted(net *lntest.NetworkHarness, t *harnessTest,
 	assertNumPendingChannels(t, carol, 0, 0)
 
 	// Make sure Carol got her balance back.
+	err = wait.NoError(func() error {
 		ctxt, _ = context.WithTimeout(ctxb, defaultTimeout)
 		carolBalResp, err := carol.WalletBalance(ctxt, balReq)
 		if err != nil {
-		t.Fatalf("unable to get carol's balance: %v", err)
+			return fmt.Errorf("unable to get carol's balance: %v", err)
 		}
+
 		carolBalance := carolBalResp.ConfirmedBalance
 		if carolBalance <= carolStartingBalance {
-		t.Fatalf("expected carol to have balance above %d, "+
+			return fmt.Errorf("expected carol to have balance "+
-			"instead had %v", carolStartingBalance,
+				"above %d, instead had %v", carolStartingBalance,
 				carolBalance)
 		}
 
+		return nil
+	}, defaultTimeout)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+
 	assertNodeNumChannels(t, dave, 0)
 	assertNodeNumChannels(t, carol, 0)
 }