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.

payments/db/kv_store_test.go

@@ -2,7 +2,6 @@ package paymentsdb
 
 import (
 	"bytes"
-	"context"
 	"math"
 	"reflect"
 	"testing"
@@ -932,7 +931,7 @@ func TestQueryPayments(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			ctx := context.Background()
+			ctx := t.Context()
 
 			paymentDB := NewKVTestDB(t)