Initial Drupal 11 with DDEV setup

web/core/tests/fixtures/test_stable/test_stable.info.yml

@@ -0,0 +1,5 @@
+name: Test Stable
+type: theme
+base theme: stable9
+description: A theme to test that stable 9 is set as the default.
+version: VERSION

web/core/tests/fixtures/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): void {
+  // This is used for Drupal\Tests\Core\Theme\RegistryTest.
+}