Our development team is working on the new feature to allow publish website to subfolder such as mycutedomainname.com/coolrvsitebuilder7 . But for the time being, you can do one in following tricks for building a test website.
1. Use a sub domain (such as test.domainname.com)
You can create a sub domain, or ask your hosting provider to create it for you for testing purpose. Once you finish and see how the website will be, you can change it to real domain name later.
2. Set as a Draft page in the real online website
If in case in a current online website, you can use Draft function. Here's the guide: https://support.rvglobalsoft.com/hc/en-us/articles/360018523874-Still-not-ready-to-publish-a-website-Just-set-it-as-a-Draft-no-one-can-see-it-
This way, you will have to create a website admin credential and send to your client to login to website admin area to see this Draft page in editor. (Because draft will not be either published or seen anywhere.)
3. Set Visibility to a sample page, in the real online website
Set page visibility and create a user role for your user to open the page and login by credential you provided. (Member Area)
Here's to use page visibility function : https://support.rvglobalsoft.com/hc/en-us/articles/360012548173-Membership-site-making-more-value-for-your-website
Here's to set user role, that you can also define your user as admin for website editor access for Draft page access in number 2: https://support.rvglobalsoft.com/hc/en-us/articles/360017896773-Assigning-role-and-grouping-your-visitors-for-more-privilege-