How can I test a service in symfony2?

If your are not going to test controllers (with client, because controller called like this will use a new container) your test class should just extend KernelTestCase (Symfony\Bundle\FrameworkBundle\Test). To boot the kernel & get services you can do sth like this in your test setup:

use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;

class MyTest extends KernelTestCase
{
  public function setUp()
  {
    $kernel = self::bootKernel();
    $this->geo = $kernel->getContainer()->get('geo');
  }
  ...
}

More info: http://symfony.com/doc/current/testing/doctrine.html

Notes:

1. To have fresh kernel & services for each test I recommend to use the setUp method instead of setUpBeforeClass (like in the accepted answer).
2. The kernel is always accessible via static::$kernel (after booting).
3. Also works in symfony 3 & 4.

In your test case, you're not going to get anything in your constructor - you can set up the object you want to test in a setupBeforeClass or setup method, e.g.

public static function setUpBeforeClass()
{
     //start the symfony kernel
     $kernel = static::createKernel();
     $kernel->boot();

     //get the DI container
     self::$container = $kernel->getContainer();

     //now we can instantiate our service (if you want a fresh one for
     //each test method, do this in setUp() instead
     self::$geo = self::$container->get('geo');
}

(Note also you don't need to set up your test classes as services either, so you can remove your geo_test service from services.yml)

Once you've done that, you should be able to run your test case with something like

cd /root/of/project
phpunit -c app src/Caremonk/MainSiteBundle/Tests/Services/GeoTest.php