[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: ba007f20bb4acf95262f49ab527ce35c4a1f5a19
11 files changed