Auto merge of #3819 - RalfJung:epoll-test, r=RalfJung

epoll test: further clean up check_epoll_wait

Given that `check_epoll_wait` compared the length of the two slices, I don't think it was possible for it to ever return `false`. It's also strange to have some requirements checked inside the function and some checked by the caller, so let's just move it all inside the function.

Cc `@tiif` -- did I miss anything?

Author: bors