I don't think there's a way to reset the entire site. The closest you'll get is to create a new blank site. But then you'll lose any customizations or custom site settings that you've made.
If you are going to need to use this "site reset" function on a regular basis then one thing you could try is to setup a new instance of Moodle and make all the configuration changes that you want - without adding any users or courses. Then make a backup of that site, including the Moodle database and Moodledata foders. Then when you want to "reset" one of your other sites you just drop the site you want to "reset" and restore it from the above backup. That will give you a new blank site, containing all the config settings you made previously.