multi: context.Background() -> t.Context()

Use the new feature of Go 1.24, fix linter warnings.

This change was produced by:
 - running golangci-lint run --fix
 - sed 's/context.Background/t.Context/' -i `git grep -l context.Background | grep test.go`
 - manually fixing broken tests
 - itest, lntest: use ht.Context() where ht or hn is available
 - in HarnessNode.Stop() we keep using context.Background(), because it is
   called from a cleanup handler in which t.Context() is canceled already.

itest/lnd_payment_test.go

@@ -1081,7 +1081,7 @@ func sendPaymentInterceptAndCancel(ht *lntest.HarnessTest,
 
 	// We initiate a payment from Alice and define the payment context
 	// cancellable.
-	ctx, cancelPaymentContext := context.WithCancel(context.Background())
+	ctx, cancelPaymentContext := context.WithCancel(ht.Context())
 	var paymentStream rpc.PaymentClient
 	go func() {
 		req := &routerrpc.SendPaymentRequest{