Auto merge of rust-lang#3891 - tiif:tokiotest, r=RalfJung

Fix tokio test ICE

Fixes rust-lang#3858

It turned out that the issue mentioned [here](rust-lang/miri#3858 (comment)) is the exact cause of ICE.
So in this PR, I changed the type of ``EpollEventInterest::epfd`` from ``i32`` to ``WeakFileDescriptionRef``.

Author: bors