test: accept ENOTEMPTY from libc++ in test-fs-rm

[codebytere]

Refs: llvm/llvm-project#197104
Refs: #57103

fs.rmSync() is implemented with std::filesystem::remove_all(). When a read-only directory has a child, libc++ before LLVM 23 drops the child's EACCES and reports the ENOTEMPTY it then gets for the parent instead (llvm/llvm-project#197104). #57103 pinned that to macOS in test-fs-rm, but it depends on the C++ standard library rather than on the OS: a Linux build against libc++ fails the read-only-subdirectory case when the suite runs as non-root, and macOS will start reporting EACCES once it ships the fixed libc++.

This accepts either code for that case on all POSIX platforms. Checked with a standalone remove_all() program over the same directory layout - libstdc++ 13 reports EACCES, libc++ 18 reports ENOTEMPTY - and by running the test as non-root against a regular (libstdc++) Linux build before and after.

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Disclosure: the code and this description were written by Claude Code, directed and reviewed by @codebytere.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (67e66b8) to head (8d19a6c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66065      +/-   ##
==========================================
- Coverage   90.27%   90.25%   -0.03%     
==========================================
  Files         789      789              
  Lines      271473   271473              
  Branches    51808    51808              
==========================================
- Hits       245066   245006      -60     
- Misses      16880    16939      +59     
- Partials     9527     9528       +1     

see 30 files with indirect coverage changes

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