PHP Tools come with bundled version of PHPUnit in case there is no user installation of PHPUnit in the project.
PHP Tools will use users' PHPUnit package in case it is configured in project'
composer.json (as a composer package). In order to use custom PHPUnit see Composer section, and ensure there is
phpunit/phpunit composer package installed.
In case there is no
phpunit/phpunit composer package, the Test Explorer will use bundled PHPUnit; a stable version of PHPUnit chosen accordingly for the PHP version configured in the Project Properties.