fts: add misc fts traversal tests

Extend fts_misc_test.c with additional test cases:

• FTS_NOCHDIR with absolute paths allows application chdir freely
• fts_name is always NUL-terminated with correct fts_namelen
• FTS_D/FTS_DP are paired and fts_level increments correctly
• FTSENT fts_errno/fts_dev/fts_ino/fts_nlink are correct
• circular symlink loop under FTS_PHYSICAL terminates
• cycle via symlink under FTS_LOGICAL yields FTS_DC
• fts_close after root deletion must not crash
• fts_close after root rename restores CWD (SVN r77497)
• FTS_NOCHDIR + empty directory does not corrupt path (SVN r49772)
• FTS_NS entry has non-zero fts_errno
• FTS_XDEV and FTS_WHITEOUT stubbed pending mount setup

Sponsored by: Google LLC (GSoC 2026)
Reviewed by: asomers, jillest
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/2248