One of the things that sets Fedora apart from other commercially backed Linux distributions is a commitment to openness and transparency that goes to the core of the operating system and the processes used to produce it. Anyone can get the tools that they need to build Fedora releases in the comfort of their own home. In this case, that tool is called pungi.
Pungi is a fairly simple tool to use. You give it the location of a kickstart file that defines what packages to include, repo definition, etc. Conveniently, such a file comes with the pungi distribution that reflects the default Fedora media. So it's really simple to just go to a terminal and do:
pungi -c /usr/share/pungi/rawhide-fedora.ks --destdir=/data --nosplitmedia --nosource
It will download the packages from a mirror, build the anaconda install image, and produce the ISO.
A little while later, you come back and have the ISO of a DVD sitting there for you, ready for your install testing!
Now, the usefulness of this tool is not just limited to building media from a pre-release tree. Say that you had an updates tree that you wanted to include. Just specify that as a repo in the kickstart file, and the updated packages will be automatically included on your install image. Or you have a specific set of packages you want to include on installable media (note that this tool is not for producing live CD's - there's another tool for that), then you can modify the ks file. Since I wanted default media for this purpose, I did not modify the ks file. However, if I were looking for media to actually install from (I do all network installs anyways), I would exclude packages that are on the media that I never use, and include packages that are not on the media that I think should be.