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Don't use subdirectories. The .htaccess files are inherited based on the filesystem placement of the directories. This means that all of your sub-sites would inherit the .htaccess from the main site which would make it a nightmare to test SEF URLs.

Instead, use subdomains. If your main site is www.example.com create a subdomain test1.example.com. Make sure that the subdomain's document root is outside the root of your main site. If your main site's root is public_html do NOT place the subdomain root inside the public_html folder (default behaviour of cPanel) because you'll have the same .htaccess inheritance problem. Instead, place it into a directory alongside public_html. I've found that the convention subdomain_html works best. So, for test1.example.com you'd create a folder named test1_html next to your public_html folder and use it as the subdomain's root.

Testing HTTPS with subdomains is fairly straightforward. Most if not all hosting providers nowadays offer integration with the free of charge Let's Encrypt certificate authority. You just need to tell your hosting control panel to use Let's Encrypt to issue and install free of charge TLS certificates for your subdomain. It takes about a minute and you have valid HTTPS. As you can see, we are even using Let's Encrypt for our site. We found it to be much easier than purchasing and installing "commercial" certificates without having any kind of compromise in security. If nothing else, Let's Encrypt is actually more secure since the certificates are issued after validating ownership of the domain whereas with commercial certificate authorities there was barely any validation (they were happy to take our money and be done with it...).