[3.10] gh-107902: gh-108948: Don't test setting suid/sgid/sticky on systems that don't support them (GH-108368) (GH-109697)

[encukou]

This skips tarfile's chmod-related tests on

• systems where setting suid/sgid is no-op (which are unsupported), and
• systems where setting the sticky bit is an OSError (like the tier-3 FreeBSD)

This also adds a few directory-related tests, which were originally omitted because they failed on some platforms. Now they will be skipped on those platforms.

In 3.11+ this was done with two commits, GH-108368 & GH-109697:

(cherry-picked from 40e52c9)
(cherry-picked from 26e06ad)

One builds on the other. IMO it makes sense to backport it all, so that these tests are the same as in 3.11+.

• Issue: tarfile doesn't set file modes correctly on FreeBSD: test_tarfile.test_modes() fails on FreeBSD #108948

• Issue: test_tarfile fails on file modes #107902