Got it!
Code:
RewriteEngine On
RewriteCond %{REQUEST_URI} !^//(.*)?$
RewriteCond %{HTTP_HOST} ^(www\.)?([^.]+)\.website\.com$ [NC]
RewriteRule ^(.*)$ //%2/$1 [QSA,L]
url: "test-a.website.com" will now load the stuff that is in "website.com/test-a/" and still show "test-a.website.com" as url
url: "test-b.website.com" will now load the stuff that is in "website.com/test-b/" and still show "test-b.website.com" as url