Mocking Repository Interface in Laravel 5

I am struggling to mock my Interface in my tests, I keep getting

UserInterface should be called exactly 1 times but called 0 times.

Example of my interface:

<?php namespace App\Contracts\User;

interface UserInterface {

    public function getAllUsers();

}

Example of my repository:

<?php namespace App\Contracts\User;

use App\User;

class UserRepository implements UserInterface {

    protected $user;

    public function __construct(User $user)
    {
        $this->user = $user;
    }

    public function getAllUsers()
    {
        return $this->user->all();
    }

And my test

<?php namespace Tests\Controllers;

use TestCase;
use Mockery;

class UserIControllerTest extends TestCase {

    public function setUp()
    {
        parent::setup();
        $this->user = Mockery::mock('App\Contracts\User\UserInterface');
    }

    public function tearDown() {
        Mockery::close();
    }

    public function testgetAllUserscall()
    {
        $mock = $this->user->shouldReceive('getAllUsers')->once()->andReturn('foo');
        $this->app->instance('App\Contracts\User\UserInterface', $mock);

        $this->call('GET', 'test');
    }

}

When I go to the route manually I get the desired result, but the mocking goes wrong. What am I missing ?