diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 87ba4a5..d180a56 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -302,7 +302,7 @@ function drupal_override_server_variables($variables = array()) {
   // Replace elements of the $_SERVER array, as appropriate.
   $request->server->replace($variables + $server_vars + $defaults);
 
-  // @todo remove once conf_path() no longer uses $_SERVER.
+  // @todo remove once Site::determinePath() no longer uses $_SERVER.
   $_SERVER = $request->server->all();
 }
 
diff --git a/core/lib/Drupal/Core/Site/Site.php b/core/lib/Drupal/Core/Site/Site.php
index 39c7057..572400e 100644
--- a/core/lib/Drupal/Core/Site/Site.php
+++ b/core/lib/Drupal/Core/Site/Site.php
@@ -145,8 +145,11 @@ public static function tearDownTest() {
     if (!isset(self::$original)) {
       throw new \RuntimeException('No original Site to revert to. Missing invocation of Site::setUpTest()?');
     }
-    if (drupal_valid_test_ua()) {
-      throw new \BadMethodCallException('Site is executing a test.');
+    // Do not allow to restore original Site singleton in a test environment,
+    // unless we are testing the test environment setup and teardown itself.
+    // @see \Drupal\simpletest\Tests\BrokenSetUpTest
+    if (drupal_valid_test_ua() && !DRUPAL_TEST_IN_CHILD_SITE) {
+      throw new \BadMethodCallException('Unable to revert Site: A test is still being executed.');
     }
     self::$instance = clone self::$original;
     self::$original = NULL;
diff --git a/core/modules/file/tests/file_test/lib/Drupal/file_test/DummyReadOnlyStreamWrapper.php b/core/modules/file/tests/file_test/lib/Drupal/file_test/DummyReadOnlyStreamWrapper.php
index d230bc5..8617566 100644
--- a/core/modules/file/tests/file_test/lib/Drupal/file_test/DummyReadOnlyStreamWrapper.php
+++ b/core/modules/file/tests/file_test/lib/Drupal/file_test/DummyReadOnlyStreamWrapper.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\file_test;
 
+use Drupal\Core\Site\Site;
 use Drupal\Core\StreamWrapper\LocalReadOnlyStream;
 
 /**
@@ -16,7 +17,7 @@
  */
 class DummyReadOnlyStreamWrapper extends LocalReadOnlyStream {
   function getDirectoryPath() {
-    return conf_path() . '/files';
+    return Site::getPath('files');
   }
 
   /**
diff --git a/core/modules/file/tests/file_test/lib/Drupal/file_test/DummyStreamWrapper.php b/core/modules/file/tests/file_test/lib/Drupal/file_test/DummyStreamWrapper.php
index cbea40f..dd919c8 100644
--- a/core/modules/file/tests/file_test/lib/Drupal/file_test/DummyStreamWrapper.php
+++ b/core/modules/file/tests/file_test/lib/Drupal/file_test/DummyStreamWrapper.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\file_test;
 
+use Drupal\Core\Site\Site;
 use Drupal\Core\StreamWrapper\LocalStream;
 
 /**
@@ -16,7 +17,7 @@
  */
 class DummyStreamWrapper extends LocalStream {
   function getDirectoryPath() {
-    return conf_path() . '/files';
+    return Site::getPath('files');
   }
 
   /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelTest.php b/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelTest.php
index 76d0fde..da7dff3 100644
--- a/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelTest.php
@@ -8,6 +8,7 @@
 namespace Drupal\system\Tests\DrupalKernel;
 
 use Drupal\Core\DrupalKernel;
+use Drupal\Core\Site\Site;
 use Drupal\Component\PhpStorage\MTimeProtectedFastFileStorage;
 use Drupal\Component\PhpStorage\FileReadOnlyStorage;
 use Drupal\simpletest\DrupalUnitTestBase;
@@ -26,6 +27,9 @@ public static function getInfo() {
   }
 
   function setUp() {
+    // Initialize the test Site singleton, so that Site::getPath() works.
+    Site::init(DRUPAL_ROOT);
+
     // DrupalKernel relies on global $config_directories and requires those
     // directories to exist. Therefore, create the directories, but do not
     // invoke DrupalUnitTestBase::setUp(), since that would set up further