Don't test setting sticky bits on files for modern BSDs

Summary: In rL297945, jhenderson added methods for setting permissions
to sys::fs, but some of the unittests that attempt to set sticky bits
(01000) on files fail on modern BSDs, such as FreeBSD, NetBSD and
OpenBSD.  This is because those systems do not allow regular users to
set sticky bits on files, only on directories.  Fix it by disabling
these particular tests on modern BSDs.

Reviewers: emaste, brad, jhenderson

Reviewed By: jhenderson

Subscribers: joerg, krytarowski, llvm-commits

Differential Revision: https://reviews.llvm.org/D32120

llvm-svn: 301220