diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -1083,6 +1083,8 @@
 function install_settings_form_validate($form, &$form_state) {
   $driver = $form_state['values']['driver'];
   $database = $form_state['values'][$driver];
+  // When testing the interactive installer, copy the database password and
+  // the test prefix.
   if ($test_prefix = drupal_valid_test_ua()) {
     $database['prefix'] = $test_prefix;
     $database['password'] = $GLOBALS['databases']['default']['default']['password'];
diff -u b/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php b/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php
@@ -78,7 +78,7 @@
     );
     $this->writeSettings($settings);
 
-    $this->drupalGet('core/install.php?langcode=en&profile=minimal');
+    $this->drupalGet($GLOBALS['base_url'] . '/core/install.php?langcode=en&profile=minimal');
     $this->drupalPost(NULL, array(), 'Save and continue');
     // Reload config directories.
     include $this->public_files_directory . '/settings.php';