[CMake] Use explicit header path when using in-tree libc++ for tests

This is a follow up to D118200 which applies a similar cleanup to
headers when using in-tree libc++ to avoid accidentally picking up
the system headers.

Differential Revision: https://reviews.llvm.org/D128035

GitOrigin-RevId: 61b410cb8b9af0aa265e730a3070b3154d869803
4 files changed