3a: Add blog page functional test

Add the new `BlogPageTest` functional test to ensure that anonymous
users can access the `/blog` page.

This fails, and returns a 404 response code rather than the expected 200
response code.

web/modules/custom/my_module/tests/src/Functional/BlogPageTest.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Drupal\my_module\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+use Symfony\Component\HttpFoundation\Response;
+
+class BlogPageTest extends BrowserTestBase {
+
+  protected $defaultTheme = 'stark';
+
+  protected static $modules = [
+    'my_module',
+  ];
+
+  /** @test */
+  public function the_blog_page_loads_for_anonymous_users_and_contains_the_right_text() {
+    $this->drupalGet('blog');
+
+    $session = $this->assertSession();
+
+    $session->statusCodeEquals(Response::HTTP_OK);
+  }
+
+}