unify cpp tests (#17947)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17947
Instead of having a gtest and a no-gtest file that you have to remember to register tests in, add a single registration point and use some macro magic to make it work for both gtest and non-gtest builds
Reviewed By: eellison
Differential Revision: D14431302
fbshipit-source-id: e1abac135992577a943eaa7abcc81a6ed31fa6e5
