I have a service that uses the EntityManagerInterface:

```
class DocFinderService
{
   protected $em;

   public function __construct(EntityManagerInterface $entityManager)
   {
     $this->em = $entityManager;
   }

  public function findDocs($specialtyCodes, $city, $state, $zip)
  {...}

```

How can I test the service using PHPUnit, specifically, how can I pass the *EntityManagerInterface* parameter from the test function?