More elf os detection cases

[williballenthin]

review after #3098
uses new ELF test files from mandiant/capa-testfiles#313

  ┌─────────────────┬────────┬──────────────────────┬───────────────────────────────────────────────────────────────┐
  │      File       │  Size  │       Purpose        │                     Strategies Exercised                      │
  ├─────────────────┼────────┼──────────────────────┼───────────────────────────────────────────────────────────────┤
  │ 9486f2c5...elf_ │ 9.4KB  │ osabi=freebsd        │ osabi (only)                                                  │
  ├─────────────────┼────────┼──────────────────────┼───────────────────────────────────────────────────────────────┤
  │ a72ac9f1...elf_ │ 7.4KB  │ osabi=openbsd        │ osabi (only)                                                  │
  ├─────────────────┼────────┼──────────────────────┼───────────────────────────────────────────────────────────────┤
  │ 5e426357...elf_ │ 1.9KB  │ osabi=hpux           │ osabi (only)                                                  │
  ├─────────────────┼────────┼──────────────────────┼───────────────────────────────────────────────────────────────┤
  │ ccb5eefc...elf_ │ 263KB  │ vdso aarch64         │ ident_directive, vdso_strings                                 │
  ├─────────────────┼────────┼──────────────────────┼───────────────────────────────────────────────────────────────┤
  │ c2b2d7ce...elf_ │ 65KB   │ linker lean          │ ph_notes, linker (only 2 signals; section headers unreadable) │
  ├─────────────────┼────────┼──────────────────────┼───────────────────────────────────────────────────────────────┤
  │ fb5cf3df...elf_ │ 5.8MB  │ go_source FreeBSD    │ osabi=freebsd, go_source=freebsd                              │
  ├─────────────────┼────────┼──────────────────────┼───────────────────────────────────────────────────────────────┤
  │ ef0ef969...elf_ │ 13.2MB │ go_buildinfo OpenBSD │ osabi, ph_notes, sh_notes, go_buildinfo=openbsd               │
  └─────────────────┴────────┴──────────────────────┴───────────────────────────────────────────────────────────────┘

Used Claude and VT to collect these files and construct the test cases.

Checklist

• No CHANGELOG update needed
• No documentation update needed
• This submission includes AI-generated code and I have provided details in the description.

[williballenthin]

[gemini-code-assist]

Code Review

This pull request introduces a new test suite in tests/test_elf_os_detection.py to validate ELF OS detection algorithms against multiple test fixtures, and updates the CHANGELOG.md to reflect these additions. The review feedback recommends defining the FIXTURES dataset directly as a native Python list of dictionaries rather than parsing a raw JSON string at runtime, which would also allow for the removal of the unused json import.

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