[libc++] Use .gen.py tests for the transitive inclusion tests

This finishes the transition of tests covered in generate_header_tests.py
to the new .gen.py format.

Differential Revision: https://reviews.llvm.org/D152008

GitOrigin-RevId: 81cc929d4f9bb8ed279b6b21284ea9a1ca7881b1
6 files changed