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.

macaroons/bake_test.go

@@ -1,7 +1,6 @@
 package macaroons_test
 
 import (
-	"context"
 	"encoding/hex"
 	"testing"
 
@@ -49,7 +48,7 @@ func TestBakeFromRootKey(t *testing.T) {
 	md := metadata.New(map[string]string{
 		"macaroon": hex.EncodeToString(macaroonBytes),
 	})
-	macCtx := metadata.NewIncomingContext(context.Background(), md)
+	macCtx := metadata.NewIncomingContext(t.Context(), md)
 
 	// The macaroon should be valid for the service, since the root key was
 	// the same.