Issue Generating Url In Unit Test

Im experiencing some issues trying to generate the url in my tests.

with the following code I got the error `Error: Call to a member function getByName() on null`

```
public function testApiIsConfirured()
{
$response = $this->json('GET',app('Dingo\Api\Routing\UrlGenerator')->version('v1')->route('api.index'));

$response->assertStatus(200);
}
```
When I tried it using the version helper i go the following error `ReflectionException: Class api.url does not exist`
```
public function testApiIsConfirured()
{
$response = $this->json('GET',version('v1')->route('api.index'));

$response->assertStatus(200);
}
```
Im using laravel:5.4