Windows native io tests are failing

[huonw]

Windows is reliably failing with

task 'io::fs::test::truncate_works::native' failed at 'receiving on a closed channel', C:\bot\slave\auto-win-32-nopt-t\build\src\libstd\comm\mod.rs:507

and

task 'io::net::unix::tests::read_eof::native' failed at 'receiving on a closed channel', C:\bot\slave\try-win\build\src\libstd\comm\mod.rs:507

Seems to correlate with #12380 landing.

[huonw]

cc @alexcrichton