Update to Google Test C++ test framework from release 1.8.0 to 1.8.1.
Replace the following files and directories from Google Test 1.8.1:
LICENSE
README.md
include/
src/
Note the LICENSE file is identical, where as the other files changed.
Also see commit which initially added Google Test:
87f7170a55
Add Google Test 1.8.0 files (#781978)