4a: Ensure that only articles are returned

Start adding a kernel test that retrieves article nodes from an
`ArticleRepository` and ensures that only articles are returned.

This fails due to a non-existent service.

web/modules/custom/my_module/tests/src/Kernel/ArticleRepositoryTest.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Drupal\Tests\my_module\Kernel;
+
+use Drupal\KernelTests\Core\Entity\EntityKernelTestBase;
+
+class ArticleRepositoryTest extends EntityKernelTestBase {
+
+  /** @test */
+  public function nodes_that_are_not_articles_are_not_returned() {
+    $repository = $this->container->get(ArticleRepository::class);
+    $articles = $repository->getAll();
+
+    $this->assertCount(3, $articles);
+  }
+
+}
+