morphOne() Polymorphic Relationship Test (User-Image) - Testing Model Relationships in Laravel

morphOne_test.php

<?php
namespace Tests\Unit;

use App\Country;
use App\Image;
use App\Post;
use App\Supplier;
use App\User;
use Illuminate\Foundation\Testing\RefreshDatabase;
use Illuminate\Foundation\Testing\WithFaker;
use Illuminate\Support\Facades\Schema;
use Tests\TestCase;

class UserTest extends TestCase
{
    use RefreshDatabase, WithFaker;

    public function setUp() :void
    {
        parent::setUp();

        $this->country = factory(Country::class)->create();
        $this->supplier = factory(Supplier::class)->create();
        $this->user = factory(User::class)->create();
        $this->post = factory(Post::class)->create();
        $this->image = factory(Image::class)->create([
          "imageable_id" => $this->user->id,
          "imageable_type" => "App\User",
        ]);
    } 

    // ...
    /** @test  */
    public function a_user_morphs_one_image()
    {
        $this->assertInstanceOf(Image::class, $this->user->image); 
    }
}