Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/modules/book/tests/modules/book_test.info.yml

@@ -0,0 +1,6 @@
+name: 'Book module tests'
+type: module
+description: 'Support module for book module testing.'
+package: Testing
+version: VERSION
+core: 8.x

core/modules/book/tests/modules/book_test.module

@@ -0,0 +1,21 @@
+<?php
+
+/**
+ * @file
+ * Test module for testing the book module.
+ *
+ * This module's functionality depends on the following state variables:
+ * - book_test.debug_book_navigation_cache_context: Used in NodeQueryAlterTest to enable the
+ *   node_access_all grant realm.
+ *
+ * @see \Drupal\book\Tests\BookTest::testBookNavigationCacheContext()
+ */
+
+/**
+ * Implements hook_page_attachments().
+ */
+function book_test_page_attachments(array &$page) {
+  if (\Drupal::state()->get('book_test.debug_book_navigation_cache_context', FALSE)) {
+    drupal_set_message(\Drupal::service('cache_contexts_manager')->convertTokensToKeys(['route.book_navigation'])->getKeys()[0]);
+  }
+}