Issue #3167034: Adding simple test coverage

core/modules/system/tests/modules/image_lazy_load_test/image_lazy_load_test.info.yml

+name: 'Image lazy load'
+type: module
+description: 'Support module for image loading attribute tests.'
+package: Testing
+version: VERSION

core/modules/system/tests/modules/image_lazy_load_test/image_lazy_load_test.routing.yml

+image_lazy_load_test:
+  path: /image-lazy-load-test
+  defaults:
+    _controller: Drupal\image_lazy_load_test\Controller\ImageLazyLoadController::renderImage
+  requirements:
+    _access: 'TRUE'

core/modules/system/tests/modules/image_lazy_load_test/src/Controller/ImageLazyLoadController.php

+<?php
+
+namespace Drupal\image_lazy_load_test\Controller;
+
+use Drupal\Core\Controller\ControllerBase;
+
+/**
+ * The ImageLazyLoadController class.
+ */
+class ImageLazyLoadController extends ControllerBase {
+
+  /**
+   * Render an image using image theme.
+   *
+   * @return array
+   *   The render array.
+   */
+  public function renderImage() {
+    return [
+      '#theme' => 'image',
+      '#uri' => '/core/themes/bartik/logo.svg',
+      '#alt' => 'Image lazy load testing image',
+      '#width' => '50%',
+      '#height' => '50%',
+    ];
+  }
+
+}

core/modules/system/tests/src/Functional/Theme/ImageLoadingAttributeTest.php

+<?php
+
+namespace Drupal\Tests\system\Functional\Theme;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests lazy loading for images.
+ *
+ * @group Theme
+ */
+class ImageLoadingAttributeTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['image_lazy_load_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests that loading attribute is enabled for images.
+   */
+  public function testImageLoadingAttribute() {
+    $this->drupalGet('image-lazy-load-test');
+    $this->assertSession()->responseContains('loading="lazy"');
+  }
+
+}