accessibilityLabel={title ? `${title}, back` : 'Go back'}
accessibilityTraits="button"
testID="header-back"

static navigationOptions = {
  headerTitle: 'Title',
  headerRight: (
    <Button
      onPress={() => alert('This is a button!')}
      title="Right"
      color="#000"
      testID='headerRightButton'
      accessibilityLabel="Right button!"
    />
  ),
  headerLeft: (
    <Button
      onPress={() => alert('This is a button!')}
      title="Left"
      color="#000"
      testID='headerLeftButton'
      accessibilityLabel="Left button!"
    />
  )
};

class MyButton extends Component {
  render() {
    return (
      <TouchableOpacity onPress={() => {}}
       testID={this.props.testID}
       accessibilityLabel={this.props.accessibilityLabel}
      >
        <View>
         // style your awesome button here
        </View>
      </TouchableOpacity>
    );
  }
}

<MyButton
 testID={'mybutton'}
 accessibilityLabel={'tap my button'} />