test: skip test_utun_ifname if unprivileged

[SteveLauC]

What does this PR do

From this post, the utun interface requires privilege, and this is exactly what happens on my mac:

$ cargo t --all-features test_utun_ifname -q

running 1 test
F
failures:

---- sys::test_sockopt::test_utun_ifname stdout ----
thread 'sys::test_sockopt::test_utun_ifname' panicked at test/sys/test_sockopt.rs:854:36:
called `Result::unwrap()` on an `Err` value: EPERM

This PR skips the test if the process is not privileged.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API