Your FuelPHP installation seems to be broken. How did you install it?
Here is how to:
$ curl get.fuelphp.com/oil | sh
$ oil create fuel_project
$ cd fuel_project
$ php composer.phar require phpunit/phpunit:3.7.* --dev
Copy fuel/packages/oil/config/oil.php
to fuel/app/config/
.
Edit it 'binary_path'
to
'binary_path' => VENDORPATH.'bin/phpunit',
For example, you can run FuelPHP Core tests:
$ oil test --group=Core