/etc/apt/sources.list.d

deb http://ftp.nl.debian.org/debian testing main

/etc/apt/preferences.d/helux

Package: *
Pin: release a=stable
Pin-Priority: 700

Package: *
Pin: release a=testing
Pin-Priority: 650

apt-get update

The upgrade will not upgrade all packages to testing due to the lower priority

apt-get upgrade