Matthias Noback has a new post today showing you how to test Symfony2 commands that use the Process component. More specifically, his tests revolve around the ones that use asynchronous assertions in the testing to remove weird behaviors that could be caused by multiple processes running them simultaneously.
Usually you would test a console command using the ConsoleTester class as described in the official Symfony documentation. But we can not use it in this case. We need to isolate the process in order for pcntl_fork() to work correctly, otherwise the PHPUnit test runner itself will be forked too, which will have the effect of running all the following unit tests twice (which can have very strange effects on the test results).
He shows how to use the Process component to start up a new process (the daemon) and check that the PID file exists. He includes an example of a "probe" to determine what processes are running and preventing them from stepping on each other.