Add a failing unit test

> Drupal\Core\DependencyInjection\ContainerNotInitializedException: \Drupal::$container is not initialized yet. \Drupal::setContainer() must be called with a real container.

web/modules/custom/example/tests/src/Unit/PostWrapperTest.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Drupal\Tests\example\Unit;
+
+use Drupal\node\Entity\Node;
+use Drupal\node\NodeInterface;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @group lessons
+ */
+final class PostWrapperTest extends UnitTestCase {
+
+  /** @test */
+  public function it_wraps_a_post(): void {
+    $node = new Node(
+      entity_type: 'post',
+      values: [],
+    );
+
+    self::assertInstanceOf(NodeInterface::class, $node);
+  }
+
+}