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/fuzz_test.go

@@ -1,7 +1,6 @@
 package macaroons
 
 import (
-	"context"
 	"testing"
 
 	"gopkg.in/macaroon-bakery.v2/bakery"
@@ -17,7 +16,7 @@ func FuzzUnmarshalMacaroon(f *testing.F) {
 
 func FuzzAuthChecker(f *testing.F) {
 	rootKeyStore := bakery.NewMemRootKeyStore()
-	ctx := context.Background()
+	ctx := f.Context()
 
 	f.Fuzz(func(t *testing.T, location, entity, action, method string,
 		rootKey, id []byte) {