Update to Drupal 8.0.5. For more information, see https://www.drupal.org/node/2679347

core/modules/system/tests/themes/test_invalid_basetheme_sub/test_invalid_basetheme_sub.info.yml

@@ -0,0 +1,6 @@
+name: 'Theme test with valid base theme but no grandparent base theme'
+type: theme
+description: 'Test theme which has a non-existent base theme in the base chain.'
+version: VERSION
+core: 8.x
+base theme: test_invalid_basetheme

core/modules/system/tests/themes/test_stable/test_stable.theme

@@ -0,0 +1,13 @@
+<?php
+
+/**
+ * @file
+ * Test preprocess functions for theme test_stable.
+ */
+
+/**
+ * Implements hook_preprocess_theme_test_render_element().
+ */
+function test_stable_preprocess_theme_test_render_element($variables) {
+  // This is used for Drupal\Tests\Core\Theme\RegistryTest.
+}