Summary:
Switch all of the uses of `<gtest/gtest.h>` to `<folly/portability/GTest.h>`.

This is painful but necessary to get the tests to compile nicely under MSVC.

Reviewed By: yfeldblum

Differential Revision: D3837300

fbshipit-source-id: 7ae43a5d5ba94c8c24fa23a485f18546416e7191