This change updates GoogleTest in base from 1.14.0 to 1.15.2.
The changes between the two versions can be found in this diff of the
two release tags:
https://github.com/google/googletest/compare/v1.14.0...v1.15.2
One notable change is that GoogleTest 1.15.x now officially requires
C++-14 (1.14.x required C++-11).
Merge commit '14f7077fed7d82046bdcbe347004132f08aba886'