Just tried to install phpunit on a new box. Obviously there's something wrong with your channel…
$ pear remote-list -c phpunit
File http://pear.phpunit.de:80/Chiara_PEAR_Server_REST/c/categories.xml not valid (received: HTTP/1.1 404 Not Found)
Just remove the channel and add it again.
Thank you. This was exactly what I needed. Just to note you don't have to delete the channel and add it again. You can simply do:
pear channel-update pear.phpunit.de