I got it to run. I had to do some more settings.
First, create a new PHP Remote Debug under the Run Configurations (The ZeroConfiguration didn't work for me). Define a Server with the name you want (here "TestServer") and enter PHPSTORM as Ide Key.
Also see the Jetbrains documentation: https://confluence.jetbrains.com/display/PhpStorm/Debugging+PHP+Web+Applications+with+Run+Debug+Configurations
Enable the option "Break at first line in PHP scripts" in under the RUN menu .
It can be possible that you need to override the file mapping. (In my case PHPStorm is not able to detect file mapping automatically) (See: https://www.jetbrains.com/help/phpstorm/10.0/override-server-path-mappings-dialog.html). It's is within same dialog where the server was created.
Start the Remote Debug Session with clicking on the corresponding icon.
Then you should be able to run PHPUnit with this command:
PHP_IDE_CONFIG="serverName=TestServer" XDEBUG_CONFIG="idekey=PHPSTORM" vendor/phpunit/phpunit/phpunit -c YOUPHPUNITXMLCONFIG
Set the PHP_IDE_CONFIG corresponding to your settings.
Set breakpoints where you want to stop.
I hope I could help you.