Update to Drupal 8.2.6. For more information, see https://www.drupal.org/project/drupal/releases/8.2.6

core/modules/simpletest/src/TestBase.php

@@ -240,7 +240,10 @@ abstract class TestBase {
   /**
    * The temporary file directory for the test environment.
    *
-   * This is set in TestBase::prepareEnvironment().
+   * This is set in TestBase::prepareEnvironment(). This value has to match the
+   * temporary directory created in install_base_system() for test installs.
+   *
+   * @see install_base_system()
    *
    * @var string
    */

core/modules/simpletest/src/Tests/WebTestBaseInstallTest.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Drupal\simpletest\Tests;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests the test-specifics customisations done in the installation.
+ *
+ * @group simpletest
+ */
+class WebTestBaseInstallTest extends WebTestBase {
+
+  /**
+   * Tests the Drupal install done in \Drupal\simpletest\WebTestBase::setUp().
+   */
+  public function testInstall() {
+    $htaccess_filename = $this->getTempFilesDirectory() . '/.htaccess';
+    $this->assertTrue(file_exists($htaccess_filename), "$htaccess_filename exists");
+  }
+
+}

core/modules/simpletest/src/WebTestBase.php

@@ -496,9 +496,15 @@ abstract class WebTestBase extends TestBase {
     $directory = DRUPAL_ROOT . '/' . $this->siteDirectory;
     copy(DRUPAL_ROOT . '/sites/default/default.settings.php', $directory . '/settings.php');
 
-    // All file system paths are created by System module during installation.
+    // The public file system path is created during installation. Additionally,
+    // during tests:
+    // - The temporary directory is set and created by install_base_system().
+    // - The private file directory is created post install by
+    //   WebTestBase::initConfig().
     // @see system_requirements()
     // @see TestBase::prepareEnvironment()
+    // @see install_base_system()
+    // @see\Drupal\simpletest\WebTestBase::initConfig()
     $settings['settings']['file_public_path'] = (object) [
       'value' => $this->publicFilesDirectory,
       'required' => TRUE,
@@ -587,15 +593,8 @@ abstract class WebTestBase extends TestBase {
   protected function initConfig(ContainerInterface $container) {
     $config = $container->get('config.factory');
 
-    // Manually create and configure private and temporary files directories.
-    // While these could be preset/enforced in settings.php like the public
-    // files directory above, some tests expect them to be configurable in the
-    // UI. If declared in settings.php, they would no longer be configurable.
+    // Manually create the private directory.
     file_prepare_directory($this->privateFilesDirectory, FILE_CREATE_DIRECTORY);
-    file_prepare_directory($this->tempFilesDirectory, FILE_CREATE_DIRECTORY);
-    $config->getEditable('system.file')
-      ->set('path.temporary', $this->tempFilesDirectory)
-      ->save();
 
     // Manually configure the test mail collector implementation to prevent
     // tests from sending out emails and collect them in state instead.