I’m not sure why this didn’t work exactly, but I decided to change my strategy since I was running short on time.
So, I quit on making plugin for tests, but rather to add it directly in Source of my project, and create test application there alongside with my game project application.
For this I had to fork and clone official Unreal Engine repository, build the engine, move my entire game project to Unreal Engine repo root, add new google test module, add target to module and other necessary files (like uproject for test app), regenerate UE project files in order for UE visual studio solution to recognize vcxproj of my google test application, run some batch scripts to generate google test application within that game project. All described in guides by Eric Lemes and github repo by Nans Pellicary.