fts: add fts regression tests

Add ATF regression tests for previously-fixed fts(3) bugs:

• PR 45723: directory with read but no execute is traversed via FTS_DONTCHDIR fallback, not silently skipped (commit 1e03bff7f2b7)
• PR 196724: FTS_SLNONE must not be returned for a non-symlink; time-bounded race test runs for 1 second with concurrent file creation/deletion (commit bf4374c54589)
• PR 262038: readdir(2) errors produce FTS_DNR with fts_errno set, not silently treated as end-of-directory (commit 0cff70ca6654)
• SVN r246641: normal traversal works correctly with O_DIRECTORY fix in fts_safe_changedir() (commit f9928f1705ee)
• SVN r261589: no crash when tree modified during traversal; time-bounded race test runs for 1 second with concurrent file creation/deletion (commit c6d38f088e5c)

Sponsored by: Google LLC (GSoC 2026)
Reviewed by: asomers
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/2257