We should use the native 'zip' extension when available (instead of PclZip) because it's faster.
See WordPress wp-admin/includes/file.php unzip_file() wrapper, and _unzip_file_ziparchive() implementation.
(In ) refs #1814 - add another test to make sure the factory method works
Anthon, can you please confirm that both use cases have been tested with auto update? is the new auto-update webtest covering one use case already maybe?
Unit tests cover the actual unzipping for both implementations.
Webtest covers the high level use case, as the underlying implementation has been abstracted.