Well that might work but I am not able to create an A record from my production box to a different IP. Only options are redirects so then my domain wont be reachable on the original production box.
But I think this works quite well what I have now.
in ispconfig3 sites tab --> add new website
as domain mydomain.com (the real one)
of course this is not reachable in the browser just the production box.
Then i go in and edit the .vhost file for the mydomain.com on my test box and change the entry ServerName to something like mydomain.testboxhostname.com and I get the test mydomain.com website. Later after my ISP has changed the A record to my test box IP i have to go back and edit the vhost file to ServerName mydomain.com or just save the settings in ispconfig3 and it will write the correct ServerName.