- Bump CI actions to latest (cache, checkout, cmake)
- Bump mimimal compiler versions
- Bump minimal test OS to Ubuntu 20.04 LTS
- gtest as submodule (version 1.12.1)
- fixes heap corruptions
- update gtest macros, include paths

Co-authored-by: Thorsten Hater <24411438+thorstenhater@users.noreply.github.com>