[libcxx] [libcxxabi] [libunwind] [runtimes][NFC] Consistently declare main() functions in tests (PR #162548)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt approved this pull request.

LGTM, only question is whether this should be `const char**` 

https://github.com/llvm/llvm-project/pull/162548