Testbuilds are builds we do approximately once a month. They are not official releases, as all the testing they get is being booted and installed once. Some of them of course run on our machines and receive more testing. They usually work just fine, there is no history of bugs or failure. Another difference to official releases is, that we also build images for currently unmaintained desktops like Gnome, Cinnamon or MATE, that are not officially released right now.