Page callback; Adds stylesheets to test theme .info.yml property processing.
\Drupal\system\Tests\Theme\ThemeInfoStylesTest
http://drupal.org/node/967266#comment-3689670
function theme_test_info_stylesheets() {
$path = drupal_get_path('module', 'theme_test');
drupal_add_css("{$path}/css/base-override.css");
drupal_add_css("{$path}/css/base-override.sub-remove.css");
drupal_add_css("{$path}/css/base-remove.css");
drupal_add_css("{$path}/css/base-remove.sub-override.css");
drupal_add_css("{$path}/css/sub-override.css");
drupal_add_css("{$path}/css/sub-remove.css");
return '';
}