diff -u b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
--- b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
@@ -19,7 +19,7 @@
    *
    * @var array
    */
-  public static $modules = array('theme_test');
+  public static $modules = array('theme_test', 'twig_theme_test');
 
   public static function getInfo() {
     return array(
@@ -77,7 +77,7 @@
     config('system.theme')
       ->set('default', 'test_theme_twig')
       ->save();
-    $this->drupalGet('theme-test/template-test');
+    $this->drupalGet('twig-theme-test/php-variables');
     foreach (_test_theme_twig_php_values() as $type => $value) {
       $this->assertRaw('<li>' . $type . ': ' . $value['expected'] . '</li>');
     }
reverted:
--- b/core/modules/system/tests/modules/theme_test/theme_test.module
+++ a/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -172,41 +172,3 @@
   return $variables['foo'];
 }
 
-/*
- * Helper function to test PHP variables in the Twig engine.
- */
-function _test_theme_twig_php_values() {
-  // Prefix each variable with "twig_" so that twig doesn't get confused
-  // between a variable and a primitive. Arrays are not tested since they should
-  // be a Drupal render array.
-  return array(
-    'twig_null' => array(
-      'value' => NULL,
-      'expected' => '',
-    ),
-    'twig_bool_false' => array(
-      'value' => FALSE,
-      'expected' => '',
-    ),
-    'twig_bool_true' => array(
-      'value' => TRUE,
-      'expected' => '1',
-    ),
-    'twig_int' => array(
-      'value' => 1,
-      'expected' => '1',
-    ),
-    'twig_int_0' => array(
-      'value' => 0,
-      'expected' => '0',
-    ),
-    'twig_float' => array(
-      'value' => 122.34343,
-      'expected' => '122.34343',
-    ),
-    'twig_string' => array(
-      'value' => 'Hello world!',
-      'expected' => 'Hello world!',
-    ),
-  );
-}
diff -u b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
--- b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
+++ b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
@@ -9,5 +9,5 @@
  * Implements THEME_preprocess_theme_test_template_test().
  */
-function test_theme_twig_preprocess_theme_test_template_test(&$variables) {
+function test_theme_twig_preprocess_twig_theme_test_php_variables(&$variables) {
   $variables['php_values'] = _test_theme_twig_php_values();
 }
reverted:
--- b/core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.html.twig
+++ a/core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.html.twig
@@ -1,8 +1,2 @@
 {# Output for Theme API test #}
 Success: Template overridden.
-
-<ul>
-{% for key, php_value in php_values %}
-  <li>{{ key }}: {{ php_value.value }}</li>
-{% endfor %}
-</ul>
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/modules/twig_theme_test/templates/twig_theme_test.php_variables.html.twig
@@ -0,0 +1,6 @@
+{# Output for Twig Theme PHP variables test #}
+<ul>
+{% for key, php_value in php_values %}
+  <li>{{ key }}: {{ php_value.value }}</li>
+{% endfor %}
+</ul>
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.info.yml
@@ -0,0 +1,6 @@
+name: 'Twig theme test'
+description: 'Support module for Twig theme system testing.'
+package: Testing
+version: VERSION
+core: 8.x
+hidden: true
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
@@ -0,0 +1,69 @@
+<?php
+
+/**
+ * Implements hook_theme().
+ */
+function twig_theme_test_theme($existing, $type, $theme, $path) {
+  $items['twig_theme_test_php_variables'] = array(
+    'template' => 'twig_theme_test.php_variables',
+  );
+  return $items;
+}
+
+/**
+ * Implements hook_menu().
+ */
+function twig_theme_test_menu() {
+  $items['twig-theme-test/php-variables'] = array(
+    'page callback' => 'twig_theme_test_php_variables_callback',
+    'access callback' => TRUE,
+    'type' => MENU_CALLBACK,
+  );
+  return $items;
+}
+
+/**
+ * Menu callback for testing php variables in a twig template.
+ */
+function twig_theme_test_php_variables_callback() {
+  return theme('twig_theme_test_php_variables');
+}
+
+/*
+ * Helper function to test PHP variables in the Twig engine.
+ */
+function _test_theme_twig_php_values() {
+  // Prefix each variable with "twig_" so that twig doesn't get confused
+  // between a variable and a primitive. Arrays are not tested since they should
+  // be a Drupal render array.
+  return array(
+    'twig_null' => array(
+      'value' => NULL,
+      'expected' => '',
+    ),
+    'twig_bool_false' => array(
+      'value' => FALSE,
+      'expected' => '',
+    ),
+    'twig_bool_true' => array(
+      'value' => TRUE,
+      'expected' => '1',
+    ),
+    'twig_int' => array(
+      'value' => 1,
+      'expected' => '1',
+    ),
+    'twig_int_0' => array(
+      'value' => 0,
+      'expected' => '0',
+    ),
+    'twig_float' => array(
+      'value' => 122.34343,
+      'expected' => '122.34343',
+    ),
+    'twig_string' => array(
+      'value' => 'Hello world!',
+      'expected' => 'Hello world!',
+    ),
+  );
+}