tasking/t.go
func (t *T) Errorf(format string, args ...interface{}) {
	fmt.Fprintf(os.Stderr, format, args...)
	t.Fail()
}
func (t *T) Errorf(format string, args ...interface{}) {
	fmt.Fprintf(os.Stderr, format, args...)
	t.Fail()
}
// Errorf is equivalent to Logf followed by Fail.