testsuite: skip symlink-dirlink-basis on platforms without RESOLVE_BENEATH

testsuite: skip symlink-dirlink-basis on platforms without RESOLVE_BENEATH

secure_relative_open() has a kernel-enforced "stay below dirfd" path
on Linux 5.6+ (openat2 RESOLVE_BENEATH) and FreeBSD 13+ (openat
O_RESOLVE_BENEATH). On Solaris, OpenBSD, NetBSD, and Cygwin the code
falls back to the per-component O_NOFOLLOW walk, which by design
rejects every directory symlink in the path -- the very case this
test exercises. Mark the test skipped there rather than have it
fail with a known regression that's tracked separately.

macOS is intentionally not in the skip list: although it does not
have O_RESOLVE_BENEATH either, the test passes there in practice;
investigation of the underlying reason is left as follow-up.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>