L5.1 Validation and Testing RestAPI

I'm building an API where other websites can send Json and call my Controller store Method. For this case they use cURL. Thats is why I can't use FormRequests. (https://laracasts.com/discuss/channels/laravel/restful-api-call-validation-and-response)

So I have a validator object in my controller like:

 private function validator($data)
    {
        return Validator::make($data, [
            'order_id' => 'required|unique:orders',
            'order_date' => 'required|date_format:"Y-m-d H:i:s"',
           //...
        ]);

How can I use my validation in my tests/OrderTest.php ?

I need something simple like this:

    public function testIsInvalidWithoutADate()
    {
        $order = factory(App\Order::class, 'noDate')->make();
    $this->assertFalse($order->validate());
    }

And what is the best practice for placing the Order rules. Should I place my rules in a static $rules variable in the order model? I don't want to list them in my OrderController validator.

Greets Krizz