[libc++] Re-enable macOS back-deployment testing

Download older roots from Dropbox instead of Green Dragon, which is too
unreliable. Also XFAIL tests that were broken for back-deployment
configurations by D98097.

Differential Revision: https://reviews.llvm.org/D99359

GitOrigin-RevId: 478d1eded229c648f2c559666d2ba19455d948cb
9 files changed