I had the same issue with Ubuntu 16.10, phpStorm 2017.2 and Laravel 5.5
Fixed it by uninstalling phpunit from my Ubuntu-system with
sudo apt-get remove phpunit
sudo apt-get install --autoremove
or for mac:
brew uninstall phpunit
brew install phpunit
My PhpStorm configuration (File -> Settings -> Languages & Frameworks -> PHP -> Test Frameworks):
Works great, now!