As I explained in #8740 :

On my computer, zombie processes might have remain in case of unit test failure.
It happend that some infinite loops were running and slowing down my system. A `ps` call alerted me that many (~100) `php -r "while (true) {}"` were running after half an hour debugging things. 

In this case, we trigger the `Process::stop` call in the `__destruct` method. I don't see any problem here, do you ?