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.

kvdb/sqlite/db_test.go

@@ -9,14 +9,13 @@ import (
 	"github.com/btcsuite/btcwallet/walletdb/walletdbtest"
 	"github.com/lightningnetwork/lnd/kvdb/sqlbase"
 	"github.com/stretchr/testify/require"
-	"golang.org/x/net/context"
 )
 
 // TestInterface performs all interfaces tests for this database driver.
 func TestInterface(t *testing.T) {
 	// dbType is the database type name for this driver.
 	dir := t.TempDir()
-	ctx := context.Background()
+	ctx := t.Context()
 
 	sqlbase.Init(0)