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

core/tests/Drupal/Tests/SessionTestTrait.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Drupal\Tests;
+
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Provides methods to generate and get session name in tests.
+ */
+trait SessionTestTrait {
+
+  /**
+   * The name of the session cookie.
+   *
+   * @var string
+   */
+  protected $sessionName;
+
+  /**
+   * Generates a session cookie name.
+   *
+   * @param string $data
+   *   The data to generate session name.
+   */
+  protected function generateSessionName($data) {
+    $prefix = (Request::createFromGlobals()->isSecure() ? 'SSESS' : 'SESS');
+    $this->sessionName = $prefix . substr(hash('sha256', $data), 0, 32);
+  }
+
+  /**
+   * Returns the session name in use on the child site.
+   *
+   * @return string
+   *   The name of the session cookie.
+   */
+  protected function getSessionName() {
+    return $this->sessionName;
+  }
+
+}