Accessing to config values

I am trying to access to config values, in order to pass a token to a constructor and be able to add it to the headers. So basically what I did was in the TestCase class, in the constructor

use Illuminate\Support\Facades\Config;

public function __construct()
{
    parent::__construct();

   $this->token = Config::get('app.token');

}

Here I am getting:

PHP Fatal error: Uncaught RuntimeException: A facade root has not been set

Also, I tried with:

   $this->token = config('app.token');

And here

PHP Fatal error:  Uncaught ReflectionException: Class config does not exist.

My other question would be, in case I can get the token, how do I can pass it to the header.

Thank you, and I hope you can undesrtand what I tried to explain.

via Chebli Mohamed