theme_page_test.module

<?php

/**
 * Implements hook_system_info_alter().
 */
function theme_page_test_system_info_alter(&$info, $file, $type) {

  // Make sure that all themes are visible on the Appearance form.
  if ($type == 'theme') {
    unset($info['hidden']);
  }
}

/**
 * Implements hook_system_theme_info().
 */
function theme_page_test_system_theme_info() {
  $themes['test_invalid_basetheme'] = drupal_get_path('module', 'system') . '/tests/themes/test_invalid_basetheme/test_invalid_basetheme.info.yml';
  $themes['test_invalid_engine'] = drupal_get_path('module', 'system') . '/tests/themes/test_invalid_engine/test_invalid_engine.info.yml';
  return $themes;
}