Ok if you have renamed or deleted the /test/ subdomain site root and wp-admin htaccess files and you are still seeing a 403 error then one of these things is the cause of the problem.
1. Most Likely: Since the 403 error is a standard/generic host server 403 error page then your web host is causing the 403 error by a restriction on your wp-admin folder. I used to see this very often in the past that web hosts were blocking access to the wp-admin folder by mistake to try and stop Brute force login attacks. You will need to contact your web host and see if that is the problem.
2. Less Likely: You have an htaccess file in your hosting account root folder that is applying its rules in the /test/ subdomain site's folder. If you have another htaccess file higher up in your folder structure /public_html/.htaccess then delete it and test things. This htaccess file will apply its rules to all subfolders/child sites below the root folder /test/, etc.
3. Less Likely: You have another plugin installed on your website that is causing the 403 error. You can rename your /plugins/ folder to /_plugins/ and test if you can access the wp-admin area.