Unit Testing Laravel: InvalidArgumentException, fail to locate factory classes (Unknown formatter)

Error: Unit-Test fail to locate factory class but running the same factory in tinker (no issue in tinker).

For Laravel 6.8 or above, changes were made to “Unit-Test” to not loaded the framework anymore. So, if you need to use a “Factory” you should create a test within the “Feature-Test” directory or:

use this (in the Unit-Test):

use Tests\TestCase;

instead of:

use PHPUnit\Framework\TestCase;

For detail, you can refer to below link:

https://github.com/laravel/framework/commit/e30a0c979d98f2f1f7b6c565e4002734237a280b

Below the error capture:

But after change the Unit-Test as below:

<?php

namespace Tests\Unit\Models;
use App\Models\Category;
////use PHPUnit\Framework\TestCase; ==> Change to below
use Tests\TestCase;

class CategoryTest extends TestCase
{
    public function test_category_has_many_children()
    {
        $childrend = Category::factory(3)->create([
             'parent_id' => $parent = Category::factory()->create(),
        ]);

        $this->assertCount(3, 
             Category::where('parent_id', $parent->id)->get()
        ); 
    }
}

The Unit-Test should be running: