[libc] Replace the use of gtest with a new light weight unittest framework.

Header files included wrongly using <...> are now included using the
internal path names as the new unittest framework allows us to do so.

Reviewers: phosek, abrachet

Differential Revision: https://reviews.llvm.org/D72743

GitOrigin-RevId: c7453fad0625be37f4b885c6cc2341fb12a66395
17 files changed