With this batch of Composer updates we have added 2.7.0, 2.7.1, 2.7.2, 2.7.3, and 2.7.4 releases as well as ALL Composer versions now come with PHPUnit included!
You will need to re-download any versions of Composer you may have if you want the PHPUnit included.
<a href="https://github.com/composer/composer/releases" target="_blank" rel="noopener">https://github.com/composer/composer/releases</a>