CMake: add gtest libs before setting warning level
CMake: add gtest libs before setting warning level

This way, we can have our abstruse high warning level without the noise
from gtest sub-project.

otris/ews-cpp,m-Schlitzer/ews-cpp,bkircher/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp,m-Schlitzer/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,otris/ews-cpp,otris/ews-cpp