The admin/tool/phpunit/cli/init.php script errors out with the latest PHPUnit, as PHPUnit does not expect to be have it's autoload.php required by anything but vendor/bin/phpunit in a composer installation. As far as I can tell, the constant PHPUNIT_COMPOSER_INSTALL should be defined before requiring PHPUnit/Autoload.php if the user is in fact running a composer install. The bin/phpunit script does that here

I don't know what the patch submission process is yet, but defining that constant in [this block|
https://github.com/moodle/moodle/blob/master/admin/tool/phpunit/cli/util.php#L51] should do it.

See also: https://github.com/sebastianbergmann/phpunit/issues/848