Add a test for https://reviews.freebsd.org/D31858 (PR 258310)

This test (based on https://github.com/jiixyj/epoll-shim/pull/32#issuecomment-891276654)
fails reproducibly on QEMU with KVM and -smp 2 prior to D31858 (committed
as 98168a6e6c12dab8f608f6b5f5b0b175d2b87ef0) and passes with the patch applied.

Reviewed By: kib, imp
Differential Revision: https://reviews.freebsd.org/D31862