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.

cluster/etcd_elector_test.go

@@ -46,7 +46,7 @@ func TestEtcdElector(t *testing.T) {
 	require.NoError(t, err)
 	t.Cleanup(cleanup)
 
-	ctx, cancel := context.WithCancel(context.Background())
+	ctx, cancel := context.WithCancel(t.Context())
 	defer cancel()
 
 	const (
@@ -70,7 +70,7 @@ func TestEtcdElector(t *testing.T) {
 	ch := make(chan *etcdLeaderElector)
 
 	wg.Add(2)
-	ctxb := context.Background()
+	ctxb := t.Context()
 
 	go func() {
 		defer wg.Done()