Intermittent panic on `old_io::process::tests::test_kill`

[Manishearth]

thread '<unnamed>' panicked at 'assertion failed: !p.wait().unwrap().success()', /Users/rustbuild/src/rust-buildbot/slave/auto-mac-64-opt/build/src/libstd/old_io/process.rs:1137
test old_io::process::tests::test_kill ... FAILED

For example, http://buildbot.rust-lang.org/builders/auto-mac-64-opt/builds/4096 failed, but the rollup containing that PR passed on the same builder

[alexcrichton]

Dupe of #19120 (listed on there), but it is indeed unfortunate :(