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

[via cfe-commits]

zeroomega 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.

That seems to fix the test:
Test run: https://ci.chromium.org/ui/p/fuchsia/builders/toolchain.ci.shadow/clang-linux-x64/b8735236964550803665/overview
based on https://github.com/zeroomega/llvm-project/tree/libunwind_fix  https://github.com/zeroomega/llvm-project/commit/1e3b4a6a97b32cd1b501ad5256036967a0821327


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