lib/libc/tests/gen: add fts_open() error and edge case tests

Add ATF test cases covering fts_open() error conditions and
edge cases:

- invalid option bits (outside FTS_OPTIONMASK) yield EINVAL
- em

lib/libc/tests/gen: add fts_open() error and edge case tests

Add ATF test cases covering fts_open() error conditions and
edge cases:

- invalid option bits (outside FTS_OPTIONMASK) yield EINVAL
- empty argv yields EINVAL
- empty path string yields FTS_NS with ENOENT
- nonexistent path yields FTS_NS, not open failure
- trailing slash does not crash (SVN r49851 regression)
- unreadable directory yields FTS_D then FTS_DNR, never FTS_DP
- multiple root paths are all visited left to right

Sponsored by: Google LLC (GSoC 2026)
Reviewed by: asomers
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/2217

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