This "kind of" does the job exept that it's showing the new URL!
ReWriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} test-a.website.com$ [NC]
RewriteRule ^(.*)$
http://website.com/test-a/index.html [R]
RewriteCond %{HTTP_HOST} test-b.website.com$ [NC]
RewriteRule ^(.*)$
http://website.com/test-b/index.html [R]