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

[SteveLauC]

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It failed in this line: https://github.com/nix-rust/nix/blob/0f4559386404f58803b27807201b4dd6d1411f50/test/sys/test_uio.rs#L48

  ---- 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.