Update to Drupal 8.1.1. For more information, see https://www.drupal.org/node/2718713

core/modules/forum/tests/src/Kernel/ForumValidationTest.php

@@ -0,0 +1,65 @@
+<?php
+
+namespace Drupal\Tests\forum\Kernel;
+
+use Drupal\KernelTests\Core\Entity\EntityKernelTestBase;
+use Drupal\node\Entity\Node;
+use Drupal\taxonomy\Entity\Term;
+
+/**
+ * Tests forum validation constraints.
+ *
+ * @group forum
+ */
+class ForumValidationTest extends EntityKernelTestBase {
+
+  /**
+   * Modules to install.
+   *
+   * @var array
+   */
+  public static $modules = ['node', 'options', 'comment', 'taxonomy', 'forum'];
+
+  /**
+   * Tests the forum validation constraints.
+   */
+  public function testValidation() {
+    // Add a forum.
+    $forum = Term::create([
+      'name' => 'forum 1',
+      'vid' => 'forums',
+      'forum_container' => 0,
+    ]);
+
+    // Add a container.
+    $container = Term::create([
+      'name' => 'container 1',
+      'vid' => 'forums',
+      'forum_container' => 1,
+    ]);
+
+    // Add a forum post.
+    $forum_post = Node::create([
+      'type' => 'forum',
+      'title' => 'Do these pants make my butt look big?',
+    ]);
+
+    $violations = $forum_post->validate();
+    $this->assertEqual(count($violations), 1);
+    $this->assertEqual($violations[0]->getMessage(), 'This value should not be null.');
+
+    // Add the forum term.
+    $forum_post->set('taxonomy_forums', $forum);
+    $violations = $forum_post->validate();
+    $this->assertEqual(count($violations), 0);
+
+    // Try to use a container.
+    $forum_post->set('taxonomy_forums', $container);
+    $violations = $forum_post->validate();
+    $this->assertEqual(count($violations), 1);
+    $this->assertEqual($violations[0]->getMessage(), t('The item %forum is a forum container, not a forum. Select one of the forums below instead.', [
+      '%forum' => $container->label(),
+    ]));
+  }
+
+}