[clang] [clang] Rewrite tests relying on shell environment variable features (PR #156904)

[Carlo Cabrera via cfe-commits]

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

Seems worth investigating adding support for `unset` in `lit`'s internal shell given how much repetition it would save here, TBH.

https://github.com/llvm/llvm-project/pull/156904