For test purpose best way as suggest would be get the installation first done on subfolder and once all the testing is done you can move the subfolder account to main site account by making few changes in Wordpress setting and file path.
Good point NKV, if plesk gives this option to select the php version required for different applications like , cubecart or say wordpress, joomla or drupal anything on which client wants to select there own php version will really help lots of web hosting providers. Thank you Peter for your...