diff --git a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
index 96bcdc8..25571b9 100644
--- a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
+++ b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
@@ -50,9 +50,15 @@ protected function prepareSettings() {
     $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
+    //   FunctionalTestSetupTrait::initConfig().
     // @see system_requirements()
     // @see TestBase::prepareEnvironment()
+    // @see install_base_system()
+    // @see \Drupal\Core\Test\FunctionalTestSetupTrait::initConfig()
     $settings['settings']['file_public_path'] = (object) [
       'value' => $this->publicFilesDirectory,
       'required' => TRUE,
diff --git a/core/modules/simpletest/src/Tests/WebTestBaseInstallTest.php b/core/modules/simpletest/src/Tests/WebTestBaseInstallTest.php
new file mode 100644
index 0000000..857e3a3
--- /dev/null
+++ b/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");
+  }
+
+}
diff --git a/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
index 7241fe4..199d257 100644
--- a/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
+++ b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
@@ -177,4 +177,12 @@ public function testCronRun() {
     $this->assertGreaterThan($last_cron_time, $next_cron_time);
   }
 
+  /**
+   * Tests the Drupal install done in \Drupal\Tests\BrowserTestBase::setUp().
+   */
+  public function testInstall() {
+    $htaccess_filename = $this->tempFilesDirectory . '/.htaccess';
+    $this->assertTrue(file_exists($htaccess_filename), "$htaccess_filename exists");
+  }
+
 }