This test works by having two post-loop events activate one another
in a tight loop. If the "post-loop" mechanism didn't work, this
would be enough to starve all other events.
Previously, you needed to store the previous log severity in a local
variable, and it wasn't clear if you were allowed to call these
functions more than once.
Only some very ancient distributions don't ship with Libevent 2 anymore,
even the oldest supported Ubuntu LTS version has it. This allows us to
get rid of a lot of compat code.
