4e: Add the getAll() method

Add the `getAll()` method to the ArticleRepository, ensuring that it
returns an array.

This returns as 0 articles are returned, but 3 are expected in the
assertion.

web/modules/custom/my_module/src/Repository/ArticleRepository.php

@@ -4,5 +4,9 @@ namespace Drupal\my_module\Repository;
 
 class ArticleRepository {
 
+  public function getAll(): array {
+    return [];
+  }
+
 }