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.

autopilot/externalscoreattach_test.go

@@ -1,7 +1,6 @@
 package autopilot_test
 
 import (
-	"context"
 	"testing"
 
 	"github.com/btcsuite/btcd/btcec/v2"
@@ -23,7 +22,7 @@ func randKey() (*btcec.PublicKey, error) {
 // ExternalScoreAttachment correctly reflects the scores we set last.
 func TestSetNodeScores(t *testing.T) {
 	t.Parallel()
-	ctx := context.Background()
+	ctx := t.Context()
 
 	const name = "externalscore"