This updates tests to track a fix applied in Mio. Previously, Mio
incorrectly fired HUP events. This was due to Mio mapping `RDHUP` to
HUP. The test is updated to correctly generate a HUP event.
Additionally, HUP events will be removed from all platforms except for
Linux. This is caused by the inability to reliably map kqueue events to
the epoll HUP behavior.