My suggestion would be to follow the examples given for you in Laravel.
Picking a random artisan test from the framework, it looks like you don't need Symfony's 'CommandTester' class, but instead you just instantiate and go.
From https://github.com/laravel/framework/blob/master/tests/Foundation/FoundationConfigPublishCommandTest.php: (link subject to become stale)
public function testCommandCallsPublisherWithProperPackageName()
{
$command = new Illuminate\Foundation\Console\ConfigPublishCommand($pub = m::mock('Illuminate\Foundation\ConfigPublisher'));
$pub->shouldReceive('publishPackage')->once()->with('foo');
$command->run(new Symfony\Component\Console\Input\ArrayInput(array('package' => 'foo')), new Symfony\Component\Console\Output\NullOutput);
}
So you instantiate your command, passing in your mocked repository interface, add your expectations to the mock, then call run
on the command, passing in 'fake' input and output classes.