Setup Debian Testing release with fallback to Unstable release

readme

Install apt-transport-https:
$ sudo aptitude install apt-transport-https

Change your /etc/apt/sources.list to be the following in order to enable testing/unstable distro:

deb https://mirrors.kernel.org/debian/ testing main contrib non-free
deb-src https://mirrors.kernel.org/debian/ testing main contrib non-free

deb https://mirrors.kernel.org/debian/ unstable main contrib non-free
deb-src https://mirrors.kernel.org/debian/ unstable main contrib non-free

deb http://security.debian.org/ testing/updates main contrib non-free
deb-src http://security.debian.org/ testing/updates main contrib non-free

Then make sure to set testing as the default preference for packages. Create the file /etc/apt/apt.conf.d/local.conf and add the following line:
APT::Default-Release "testing";

Then update and upgrade:
$ sudo aptitude update
$ sudo aptitude full-upgrade

Check if a package has an unstable version available:
$ aptitude versions package_name

local.conf

APT::Default-Release "testing";

sources.list

deb https://mirrors.kernel.org/debian/ testing main contrib non-free
deb-src https://mirrors.kernel.org/debian/ testing main contrib non-free

deb https://mirrors.kernel.org/debian/ unstable main contrib non-free
deb-src https://mirrors.kernel.org/debian/ unstable main contrib non-free

deb http://security.debian.org/ testing/updates main contrib non-free
deb-src http://security.debian.org/ testing/updates main contrib non-free