Unable to test Symfony2 console command

Question

As part of learning Symfony2, I'm trying to write a very simple console command that simply runs phpcs (PHP Code Sniffer).

Here's the execute function which is in a class extending ContainerAwareCommand:

protected function execute(InputInterface $input, OutputInterface $output)
{
    $output->writeln('<info>Generating PHP Code Sniffer report...</info>');
    exec('phpcs ./src > ./app/logs/phpcs.log');

    if ($input->getOption('noprompt') == null) {
        $dialog = $this->getHelperSet()->get('dialog');
        if ($dialog->askConfirmation($output, '<question>Open report in TextMate? (y/n)?</question>', false)) {
            exec('mate ./app/logs/phpcs.log');
        }
    }

    $output->writeln('<info>...done</info>');
}

I am able to execute the console command by running

app/console mynamespace:ci:phpcs

and it works perfectly. The output file is generated as expected.

I'm trying to test the mynamespace:ci:phpcs command using the following function (which is part of a PHPUnit_Framework_TestCase):

public function testExecute()
{
    $kernel = new \AppKernel("test", true);
    $kernel->boot();

    $application = new Application($kernel);
    $application->add(new PhpCodeSnifferCommand());

    $command = $application->find('mynamespace:ci:phpcs');
    $commandTester = new CommandTester($command);
    $commandTester->execute(array('command' => $command->getName()));

    // ... Test if output file was created here ... ommitted for brevity ... //
}

However, when trying to execute it via the unit test, it fails with the following output:

sh: phpcs: command not found

Does anyone have an idea why this is happening?

PS: One thing I did observe was, that when I comment out the lines in the command that call 'exec' the test runs through (not passing, but not moaning that phpcs doesn't exist), so the problem is definitely with the exec commands.

Does the PHPUnit tests run as a different user, where phpcs is not available?