Laravel 8 create user to be used for entire test class

I have a laravel 8 project. I want to create a new user using factory with specific role e.g. admin, to be used for the entire class of tests. I am trying to do this using setUpBeforeClass().

Here is what I have so far:

PostPagesTest.php

<?php

namespace Tests\Feature;

use Illuminate\Foundation\Testing\RefreshDatabase;
use Illuminate\Foundation\Testing\WithFaker;
use Tests\TestCase;

use App\Models\User;

class PostPagesTest extends TestCase
{

    protected $adminUser;

    public static function setUpBeforeClass(): void
    {
        parent::setUpBeforeClass();
        self::$adminUser = User::factory()->withRole('admin')->create();  // works under tinker
    }

    public function test_index_page_visible_as_admin()
    {
        $response = $this->actingAs($this->adminUser)
            ->get('/posts')
            ->assertSeeText('Posts')
            ->assertStatus(200);
    }

    public function test_create_page_visible_as_admin()
    {
        $response = $this->actingAs($this->adminUser)
            ->get('/posts/create')
            ->assertStatus(200);
    }
}

Unfortunately, when I run the test using php artisan test --filter=PostPagesTest I get the following error:

  Call to a member function connection() on null

  at C:\project\vendor\laravel\framework\src\Illuminate\Database\Eloquent\Model.php:1653
    1649▕      * @return \Illuminate\Database\Connection
    1650▕      */
    1651▕     public static function resolveConnection($connection = null)
    1652▕     {
  ➜ 1653▕         return static::$resolver->connection($connection);
    1654▕     }
    1655▕
    1656▕     /**
    1657▕      * Get the connection resolver instance.

Any ideas on how I can do this i.e. create an admin user and use it through my tests.