Im interested how could it be possible to build testing VMs?
I think there is not much info about it or maybe I wrong?
It is not officially supported. Because there is no maintainer to support that use case. (Maintainer means here, someone who checks that the build script works, checks if the resulting images are sane, answers questions in the forums, eventually fixes bugs.)
I personally don’t support it, because testing is a too fast moving target. Whonxi 7 was based on Debian testing and this really was non-ideal. Issues coming up every now and then. No anonymity/security issues, but grave usability bugs.
There is a build option.
(Briefly tested it before release of Whonix 9.) But it really is developers-only at the moment.
(By using variables, one could also build using stable current sources, but these builds are more likely to break.)
Work on jessie will likely at latest start when jessie get the new Debian stable, but more likely after the freeze of jessie.