Вы захотите что-то вроде этого:
RewriteEngine On
# Only redirect if we're on the subdomain and haven't redirected
# (internally) to /www/ yet
RewriteCond %{HTTP_HOST} ^test
RewriteCond %{REQUEST_URI} !^/www/
RewriteRule ^.*$ /www/$0 [L]
# Let's also prevent them from being able to go to our /www path manually
RewriteCond %{THE_REQUEST} ^(POST|GET)\s/www
RewriteRule ^www/?(.*)$ http://test.mydomain.com/$1 [R,L]