sys::test_aio::test_aio_cancel_all segfaults on i686-musl

[asomers]

Recent Travis runs have begun to segfault during test_aio_cancel_all for i686-unknown-linux-musl. japaric/cross hasn't had a new release in about 3 months. Travis updated its Ubuntu 18.04 image lately (https://changelog.travis-ci.com/ubuntu-18-04-bionic-os-improvements-131454), but we aren't using it. Perhaps an unannounced kernel update triggered the failure? In any case, I can't reproduce the problem locally and musl has long been problematic. This thread suggests that such failures are due to a stack overflow within musl. I'm just going to skip the test.

https://www.openwall.com/lists/musl/2018/12/12/2
https://travis-ci.org/nix-rust/nix/jobs/628509821?utm_medium=notification&utm_source=github_status
https://travis-ci.org/nix-rust/nix/jobs/628361673?utm_medium=notification&utm_source=github_status
https://travis-ci.org/nix-rust/nix/jobs/625253951?utm_medium=notification&utm_source=github_status