First of all, you need PEAR to install PHPUnit.
pacman -S php-pear
Now you can do just like the official documentation. As root, just type
pear channel-discover pear.phpunit.de pear channel-discover components.ez.no pear channel-discover pear.symfony-project.com pear install phpunit/PHPUnit
For running PHPUnit properly you must have the /usr/bin/ directory listed in the open_basedir option at the /etc/php/php.ini
open_basedir = /srv/http/:/home/:/tmp/:/usr/share/pear/:/usr/bin/
Or just unset it