Previously, Drupal simpletests were executed using the same "sites/*" folder as the host installation, relying on a series of hacks and overrides to simulate a clean environment.
Simpletests now leverage Drupal's native multi-site functionality to construct a separate environment for test runs.
If you wish to run simpletests on your Drupal site, you must have a server-writable /sites/simpletest directory. Simpletest will try to create this directory automatically. If it is unable to do so, you will have to create the directory manually.
As the actual site settings.php and other files are in subdirectories of the sites/simpletest directory, the simpletest sites do not conflict with actual sites of a multi-site installation. To prevent execution of the server written PHP files from public HTTP requests, a .htaccess file is generated similar to that in the public files directory.
