Issue #3230507: Create build tests for case where site is using core recommended Composer project

tests/src/Build/AttendedCoreRecommendedUpdateTest.php

+<?php
+
+namespace Drupal\Tests\automatic_updates\Build;
+
+/**
+ * Tests an end-to-end core update via the core-recommended metapackage.
+ *
+ * @group automatic_updates
+ */
+class AttendedCoreRecommendedUpdateTest extends AttendedCoreUpdateTest {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $docRoot = 'docroot/';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function prepareForUpdate(string $version): void {
+    parent::prepareForUpdate($version);
+
+    $composer = $this->getWorkspaceDirectory() . "/composer.json";
+    $data = $this->readJson($composer);
+    $data['repositories']['drupal/core-recommended'] = [
+      'type' => 'path',
+      'url' => $this->createTargetMetapackage($version),
+      'options' => [
+        'symlink' => FALSE,
+      ],
+    ];
+    $this->writeJson($composer, $data);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function test(): void {
+    $this->markTestSkipped('This test is disabled until automatic_updates supports drupal/core-recommended.');
+  }
+
+  /**
+   * Creates a mocked version of drupal/core-recommended to update to.
+   *
+   * @param string $version
+   *   The version of the mocked metapackage.
+   *
+   * @return string
+   *   The path of the mocked package.
+   */
+  protected function createTargetMetapackage(string $version): string {
+    $destination = $this->mockPackage($this->getDrupalRoot() . '/composer/Metapackage/CoreRecommended');
+
+    $composer = $destination . '/composer.json';
+    $data = $this->readJson($composer);
+    $data['version'] = $version;
+    $data['require']['drupal/core'] = $version;
+    $this->writeJson($composer, $data);
+
+    return $destination;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getComposerConfiguration(): array {
+    $configuration = parent::getComposerConfiguration();
+
+    $require = &$configuration['require'];
+    $require['drupal/core-recommended'] = $require['drupal/core'];
+    unset($require['drupal/core']);
+
+    return $configuration;
+  }
+
+}