tests: allow using system gtest copy
tests: allow using system gtest copy

Distributions often do builds with no network access available
for both security reasons and also to ensure reproducibility.

This change tells CMake to query the system for a copy of gtest,
but if it's not available, it'll fall back to downloading via
FetchContent.

serge1/ELFIO,serge1/ELFIO,serge1/ELFIO