Test `sys::test_uio::test_writev` randomly fails on Solaris

[SteveLauC]

It failed in this line:

nix/test/sys/test_uio.rs

Line 48 in 0f45593

let written = write_res.expect("couldn't write");

  ---- sys::test_uio::test_writev stdout ----
  thread 'sys::test_uio::test_writev' panicked at test/sys/test_uio.rs:48:29:
  couldn't write: EINVAL

Currently, we have no idea about the root cause so we decided to disable it.