Yes `RUN_ONCE` is probably the reason why it doesn't fail anymore in that condition.

By the way if we run the test suite in strict mode (which requires [`phpunit/php-invoker`](https://packagist.org/packages/phpunit/php-invoker) and consequently `ext-pcntl`), each test case must complete within a certain timeout (the default value is 1 second) or it gets "killed" and marked as failed. The only minor downside is that on some recent Ubuntu distributions `pcntl` functions are explicitly disabled via `disable_functions` in `php.ini`, but considered the very nature of this project and how tests could fail it's probably better to run in strict mode anyway.

Let's leave this for later.