diff -u b/core/modules/simpletest/src/InstallerTestBase.php b/core/modules/simpletest/src/InstallerTestBase.php
--- b/core/modules/simpletest/src/InstallerTestBase.php
+++ b/core/modules/simpletest/src/InstallerTestBase.php
@@ -137,25 +137,6 @@
 
     // Configure site.
     $this->setUpSite();
-
-    // Import new settings.php written by the installer.
-    $request = Request::createFromGlobals();
-    $class_loader = require $this->container->get('app.root') . '/autoload.php';
-    Settings::initialize($this->container->get('app.root'), DrupalKernel::findSitePath($request), $class_loader);
-    foreach ($GLOBALS['config_directories'] as $type => $path) {
-      $this->configDirectories[$type] = $path;
-    }
-
-    // After writing settings.php, the installer removes write permissions
-    // from the site directory. To allow drupal_generate_test_ua() to write
-    // a file containing the private key for drupal_valid_test_ua(), the site
-    // directory has to be writable.
-    // WebTestBase::tearDown() will delete the entire test site directory.
-    // Not using File API; a potential error must trigger a PHP warning.
-    chmod($this->container->get('app.root') . '/' . $this->siteDirectory, 0777);
-    $this->kernel = DrupalKernel::createFromRequest($request, $class_loader, 'prod', FALSE);
-    $this->kernel->prepareLegacyRequest($request);
-    $this->container = $this->kernel->getContainer();
   }
 
   /**
@@ -193,6 +174,25 @@
     $edit = $this->translatePostValues($this->parameters['forms']['install_configure_form']);
     $this->drupalPostForm(NULL, $edit, $this->translations['Save and continue']);
 
+    // Import new settings.php written by the installer.
+    $request = Request::createFromGlobals();
+    $class_loader = require $this->container->get('app.root') . '/autoload.php';
+    Settings::initialize($this->container->get('app.root'), DrupalKernel::findSitePath($request), $class_loader);
+    foreach ($GLOBALS['config_directories'] as $type => $path) {
+      $this->configDirectories[$type] = $path;
+    }
+
+    // After writing settings.php, the installer removes write permissions
+    // from the site directory. To allow drupal_generate_test_ua() to write
+    // a file containing the private key for drupal_valid_test_ua(), the site
+    // directory has to be writable.
+    // WebTestBase::tearDown() will delete the entire test site directory.
+    // Not using File API; a potential error must trigger a PHP warning.
+    chmod($this->container->get('app.root') . '/' . $this->siteDirectory, 0777);
+    $this->kernel = DrupalKernel::createFromRequest($request, $class_loader, 'prod', FALSE);
+    $this->kernel->prepareLegacyRequest($request);
+    $this->container = $this->kernel->getContainer();
+
     // Manually configure the test mail collector implementation to prevent
     // tests from sending out e-mails and collect them in state instead.
     $this->container->get('config.factory')