Is it possible to obtain testing releases of elementary OS ISO images? Otherwise - are there any repos for the latest unstable/testing/etc. packages? How do I add them here?
You can try the daily PPA which contains unstable pre release software. To add it run the following commands
sudo add-apt-repository ppa:elementary-os/daily
sudo apt update && sudo apt dist-upgrade
Since this gives you bleeding-edge daily builds, your system is most likely going to be broken if you do this.