code: func (m *Mock) fail(format string, args ...interface{}) {
  	m.mutex.Lock()
  	defer m.mutex.Unlock()
  
  	if m.test == nil {
  		panic(fmt.Sprintf(format, args...))
  	}
  	m.test.Errorf(format, args...)
  	m.test.FailNow()
  }
docs: // fail fails the current test with the given formatted format and args.
  // In case that a test was defined, it uses the test APIs for failing a test,
  // otherwise it uses panic.