5a: Create a mock article

As this is a unit test, we don’t have access to a database or the
service container, so we will need to use a mock.

This is enough to pass the `instanceOf` assertion, but the others are
failing as those methods are returning `null`.

web/modules/custom/my_module/tests/src/Unit/Wrapper/ArticleWrapperTest.php

@@ -10,6 +10,8 @@ class ArticleWrapperTest extends UnitTestCase {
 
   /** @test */
   public function it_can_return_the_article() {
+    $article = $this->createMock(NodeInterface::class);
+
     $articleWrapper = new ArticleWrapper($article);
 
     $this->assertInstanceOf(NodeInterface::class, $articleWrapper->getOriginal());