feature-2912

[+] Add tests to build

[-] some "add_directory", that breakes cmake

[-] Another add_subdirectory that is do nothing but breaks cmake

[+] Add include guard to OS_detection

[*] Bump CMake minimum version for that

[*] Turn on tests building

[*] Fix sign_data unpacking for ringct20 in test

[+] add new tests from original implementation to ringct20