I created a test branch sub-domain https://test.articles.foletta.org, however I’m getting a certificate error as it’s presenting a *.netlify.com Common Name. I can’t ignore the error as it’s using HSTS.
I checked out your site and see that you still need to do 2 things: deploy the test branch on Netlify and create the subdomain on Netlify. You have another working branch subdomain on that site set up correctly, so the new branch subdomain will just work just as well once its created.