If this happens and you see the red I with PHPUnit Version not installed then you probably get the message "Please select PHP Interpreter to load PHPUnit version" when you press the refresh button.
If you have the path to phpunit.phar
set and your path the composer autoloader is fine, then it is possible that PHPStorm simply doesn't get what PHP exe file to use to load the PHPUnit.
You can then choose to add a Remote PHP Interpreter but you may also choose to add a local PHP Interpreter (for instance if you have XAMPP or WAMPP or LAMPP installed on your computer).
Press Ctrl+Alt+S and go to -> Languages & Frameworks -> PHP -> CLI Interpreter -> go for the button with the dots (or shift + enter) and there you can add an "Interpreter" by pressing the + button.
I added my own local interpreter xampp\php\php.exe
Press apply and okay and go back to
Languages & Frameworks -> PHP -> Test Frameworks and go for the refresh button.
It should do some magic and then it will say PHPUnit version: 6.5 (or whichever you installed).
I had my local (XAMPP) Apache server running.