Laravel unit testing how to mock eloquent's query scope method

I would like to mock model's scope method, is it possible? How?

Here is the scenario:

Order model:

public function scopeForUser($query) // query scope method which should be mocked
{

    return $query->where('user_id', 1);
}

User model:

public function getOrders()
{

    $orders = $this->orders(); // Returns Eloquent object(HasMany) 

    $orders->forUser(); // I would like to mock forUser() method

    return $orders->get();
}

TestCase:

public function testGetUsersOrders()
{
    $userOrders = Order::where('user_id', 1);

    // Mock Order's forUser method
    $this->mock =  Mockery::mock('Eloquent', 'Order'); // ?? I can't mock HasMany object, can I?
    $this->app->instance('Order', $this->mock);

    $this->mock->shouldReceive('forUser')->once()->andReturn($userOrders->get());

    $result = $this->user->getOrders();
    $this->assertEquals($userOrders->get(), $result);

}

This is simplified example.