Unittesting a Symfony 4.2 process runs infinite loop (than times out), wihout unittest it works fine

Lets say I have the following Symfony 4 command:

class Command1  extends Command {
    protected static $defaultName = 'app:command1';

    protected function execute(InputInterface $input, OutputInterface $output){
        $process = new Process('bin/console list', getcwd());
        $process->start(); // or even $process->run() does not matter if its async or not
        // ... where we handle if its finished, etc...
    }
}

If I simply call bin/console app:command1 it will return the expected command list. Basically works as I expect.

But if I have a phpunit test which uses the Symfony\Component\Console\Application::run() to start this command, I end up in an "infinite loop" (well, actually not, it times out after 60 sec) in the Symfony\Component\Process::wait() in the

do {
    $this->checkTimeout();
    $running = '\\' === \DIRECTORY_SEPARATOR ? $this->isRunning() : $this->processPipes->areOpen();
    $this->readPipes($running, '\\' !== \DIRECTORY_SEPARATOR || !$running);
} while ($running);

where the $this->processPipes->areOpen() will be always open.

It seems to me, if I use any Symfony console command in a Process through phpunit, there will be always two pipes open like these:

1 = {resource} resource id='x' type='stream'
2 = {resource} resource id='z' type='stream'

but I don't know what are these actually. I also saw in htop, that the start()'s proc_open actually starts up a new process, but it just hangs (does absolutely nothing, cant even debug it), until times out. Nothing in error logs (other than the timeout).