Save Scum Method has now been patched (26/02/23). I'll leave the video below for anyone who might be curious what that was.
Theres a total of 8 Testing Grounds to be found throughout Atomic Heart. All 8 will be marked on your map, once you've completed the main story questline "Morning Express" - travelling to the next open world area by train.
A Testing Ground consists of 2-3 large puzzle rooms, which get progressively harder as you go through them. Upon completing each room you'll be rewarded with a Bronze, Silver and Gold chest (Lootyagin). In some of the Testing Grounds you may encounter a "Combat" room where you need to kill all enemies before moving on.
See my full walkthrough guide below for all 8 Testing Ground locations, entry, puzzles and Lootyagin chests:Old Method: