How to troubleshoot subdomain issue on Aegir
Your platform is most likely missing the .htaccess file in its top-level directory, or (less likely, or else the Aegir interface itself would likely show the same problem) mod_rewrite is not enabled on your site.

See if you can get to http://testname.mydomain.com/?q=user

If that works, but http://testname.mydomain.com/user doesn't, then your .htaccess file is missing or being ignored by Apache.