dependency_injection_CI.php

<?php

require_once "../vendor/autoload.php";
//Dentro de um arquivo qualquer que você for carregar no bootstrap do CI, como o config.php e routes.php
//Exemplo: require_once "dependency_injection_CI.php";

//O composer irá se encarregar de carregar
use Projeto\Empresa\Services\ProductManager;

//Singleton do CI
$CI =& get_instance();

$container = new Pimple();
$container['product_manager'] = function ($c) use ($CI) {
    $CI->load->model("product_model");
    
    return new ProductManager($CI->product_model);
}

//Agora o container do Pimple é acessível em qualquer lugar
$CI->set_item('container', $container);

ProductManager.php

<?php

namespace Projeto\Empresa\Services;

class ProductManager
{
    private $productDAO;
    
    public function __construct(\Product_model $productDAO)
    {
        $this->productDAO = $productDAO;
    }
    
    public function doSomething()
    {
        return true;
    }
}

ProductManagerTest.php

<?php
class ProductManagerTest extends PHPUnit_Framework_TestCase
{
    private $container;
    
    public function setUp()
    {
        $CI =& get_instance();
        $this->container = $CI->config->item("container");
    }
    
    public function testDoSomethingShouldWork()
    {
        //O pimple carrega por lazy load as dependencias do ProductManager (no caso o model)
        $productManager = $this->container['product_manager'];
        
        $this->assertTrue($productManager->doSomething());
    }
}

dependency_injection_CI.php line 21:
$CI->config->set_item('container', $container);