Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/behat/mink/driver-testsuite/tests/Basic/ScreenshotTest.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace Behat\Mink\Tests\Driver\Basic;
+
+use Behat\Mink\Tests\Driver\TestCase;
+
+class ScreenshotTest extends TestCase
+{
+    public function testScreenshot()
+    {
+        if (!extension_loaded('gd')) {
+            $this->markTestSkipped('Testing screenshots requires the GD extension');
+        }
+
+        $this->getSession()->visit($this->pathTo('/index.html'));
+
+        $screenShot = $this->getSession()->getScreenshot();
+
+        $this->assertInternalType('string', $screenShot);
+        $this->assertFalse(base64_decode($screenShot, true), 'The returned screenshot should not be base64-encoded');
+
+        $img = imagecreatefromstring($screenShot);
+
+        if (false === $img) {
+            $this->fail('The screenshot should be a valid image');
+        }
+
+        imagedestroy($img);
+    }
+}