Comment module's node links' front-end perf-optimizing drupalSettings are missing

Problem/Motivation

This is a bug that got introduced somewhere along the way. #2151459: Enable node render caching was the patch that originally introduced this:

+      if ($view_mode == 'teaser' && \Drupal::moduleHandler()->moduleExists('history') && \Drupal::currentUser()->isAuthenticated()) {
+        $node_links['comment__' . $field_name]['#attached']['library'][] = array('comment', 'drupal.node-new-comments-link');
+
+        // Embed the metadata for the "X new comments" link (if any) on this node.
+        $node_links['comment__' . $field_name]['#post_render_cache']['history_attach_timestamp'] = array(
+          array('node_id' => $node->id()),
+        );
+        $node_links['comment__' . $field_name]['#post_render_cache']['Drupal\comment\CommentViewBuilder::attachNewCommentsLinkMetadata'] = array(
+          array('entity_type' => $node->getEntityTypeId(), 'entity_id' => $node->id(), 'field_name' => $field_name),
+        );
+      }

… but no test coverage was included. These #post_render_cache callbacks are currently not being invoked at all. Fortunately, the only consequence of that is worse front-end performance, because we need to make requests to get that data now.

Proposed resolution

Fix it.

Remaining tasks

Write test coverage.

User interface changes

None.

API changes

None.