Jon,
From the looks of it, you may be invoking the tests from your host OS, not
the Vagrant-managed VM. Trying logging in to the VM using `vagrant ssh` and executing the tests from there.
master x ~/git/vagrant $ vagrant ssh ... vagrant@mediawiki-vagrant:~$ cd /vagrant/mediawiki/tests/phpunit vagrant@mediawiki-vagrant:/vagrant/mediawiki/tests/phpunit$ php phpunit.php
If you still have problems with it, feel free to come grab me and we can troubleshoot it further.
On a related note, I'll be working on improving the mediawiki-vagrant browser tests setup for MobileFrontend in the coming weeks. It'd be great to have you, or someone else on the mobile team, vet the improvements.
Cheers, Dan