diff -u b/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
--- b/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -295,11 +295,14 @@
    * when needed.
    */
   function testSuggestionPreprocessForDefaults() {
+    $this->config('system.theme')
+      ->set('default', 'test_theme')
+      ->save();
     // Test with both an unprimed and primed theme registry.
     drupal_theme_rebuild();
     for ($i = 0; $i < 2; $i++) {
       $this->drupalGet('theme-test/preprocess-suggestions');
-      $this->assertText('Theme hook implementor=test_theme_theme_test_preprocess__suggestion(). Foo=template_preprocess_theme_test_preprocess', 'Theme hook ran with data available from a preprocess function for the suggested hook.');
+      $this->assertText('Theme hook implementor=test_theme_preprocess_theme_test_preprocess_suggestions__suggestion(). ', 'Theme hook ran with data available from a preprocess function for the suggested hook.');
     }
   }
 }
only in patch2:
unchanged:
--- a/core/modules/system/src/Tests/Theme/RegistryTest.php
+++ b/core/modules/system/src/Tests/Theme/RegistryTest.php
@@ -75,42 +75,6 @@ function testRaceCondition() {
   }
 
   /**
-   * Tests the theme registry with multiple subthemes.
-   */
-  public function testMultipleSubThemes() {
-    $theme_handler = \Drupal::service('theme_handler');
-    $theme_handler->install(['test_basetheme', 'test_subtheme', 'test_subsubtheme']);
-
-    $registry_subsub_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_subsubtheme');
-    $registry_subsub_theme->setThemeManager(\Drupal::theme());
-    $registry_sub_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_subtheme');
-    $registry_sub_theme->setThemeManager(\Drupal::theme());
-    $registry_base_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_basetheme');
-    $registry_base_theme->setThemeManager(\Drupal::theme());
-
-    $preprocess_functions = $registry_subsub_theme->get()['theme_test_template_test']['preprocess functions'];
-    $this->assertIdentical([
-      'template_preprocess',
-      'test_basetheme_preprocess_theme_test_template_test',
-      'test_subtheme_preprocess_theme_test_template_test',
-      'test_subsubtheme_preprocess_theme_test_template_test',
-    ], $preprocess_functions);
-
-    $preprocess_functions = $registry_sub_theme->get()['theme_test_template_test']['preprocess functions'];
-    $this->assertIdentical([
-      'template_preprocess',
-      'test_basetheme_preprocess_theme_test_template_test',
-      'test_subtheme_preprocess_theme_test_template_test',
-    ], $preprocess_functions);
-
-    $preprocess_functions = $registry_base_theme->get()['theme_test_template_test']['preprocess functions'];
-    $this->assertIdentical([
-      'template_preprocess',
-      'test_basetheme_preprocess_theme_test_template_test',
-    ], $preprocess_functions);
-  }
-
-  /**
    * Tests that the theme registry can be altered by themes.
    */
   public function testThemeRegistryAlterByTheme() {