Enabling the Take web site offline for maintenance (Setup > Site Builder > Set Up Web Site) will make all users, including admins, who access the site will see the maintenance message.
To be able for a user to test their deployments, it would be best to test them using the following:
- Using a Sandbox account for testing.
Note: Sandbox accounts needs to be purchased.
- Users can use the Multiple Web Sites feature to create another web site solely for testing.
Note: This needs to be purchased and enabled on the account.