gtest learning

Example command to compile: g++ -Wall -Werror --std=c++11 ./01_add_function_gtest.cpp -lgtest -lgtest_main -lpthread

Link for installation:
https://stackoverflow.com/questions/13513905/how-to-setup-googletest-as-a-shared-library-on-linux

Basic example:
https://www.eriksmistad.no/getting-started-with-google-test-on-ubuntu/