[libc++abi] Add a from-scratch testing config for Apple backdeployment

We added one for libc++ recently, and this patch adds one for libc++abi.
Also, as a fly-by fix, include older libunwind dylibs in the testing of
libc++ and libc++abi, which fixes some issues related to running
back-deployment tests on newer systems.

Differential Revision: https://reviews.llvm.org/D119466