[libcxx] [libcxxabi] [libunwind] [libc++abi][libunwind] Run c++abi and unwind tests against a fake install root (PR #110171)

[Louis Dionne via cfe-commits]

ldionne wrote:

Thanks for the analysis. Per-target runtime dir strikes again! That setting keeps on creating issues (by no fault of its own) because it's not the default.

Can we do something like this instead, then?

```
config.substitutions.append(('%{lib}', '@LIBUNWIND_TESTING_INSTALL_PREFIX@/@LIBUNWIND_INSTALL_LIBRARY_DIR@'))
```

We should fix this one forward since there's a complicated dependency chain on top of this (we switched to Github-hosted runners for macOS testing after this patch and they will start failing if we revert this). Hopefully it's not too complicated to fix forward.

https://github.com/llvm/llvm-project/pull/110171