// start code
// src/Repository/ArticleNodeRepository.php

namespace Drupal\drupalcon\Repository;

final class ArticleRepository {
}
// end code

---

// start services
# drupalcon.services.yml

services:
  Drupal\drupalcon\Repository\ArticleRepository: ~
// end services

---

namespace Drupal\Tests\drupalcon\Kernel;

use Drupal\drupalcon\Repository\ArticleRepository;
use Drupal\KernelTests\Core\Entity\EntityKernelTestBase;

class ArticleRepositoryTest extends EntityKernelTestBase {

// start test
  public static $modules = [
    'drupalcon',
  ];

  /** @test */
  public function it_returns_blog_posts() {
    /** @var ArticleRepository */
    $repository = $this->container->get(ArticleRepository::class);

    $articles = $repository->getAll();

    $this->assertCount(1, $articles);
  }
// end test

}

// start output
E                                                                   1 / 1 (100%)

Time: 00:00.403, Memory: 6.00 MB

There was 1 error:

1) Drupal\Tests\drupalcon\Kernel\ArticleRepositoryTest::it_returns_blog_posts
Error: Call to undefined method Drupal\drupalcon\Repository\ArticleRepository::getAll()

/app/web/modules/custom/drupalcon/tests/src/ArticleRepositoryTest.php:18
/app/vendor/phpunit/phpunit/src/Framework/TestResult.php:728

ERRORS!
Tests: 1, Assertions: 4, Errors: 1.
// end output