Add initial simple threadpool test.

This one simply pushes a task into the threadpool and ensures
that the listener gets the callbacks expected.

It currently crashes, so I need to figure out what's wrong.



git-svn-id: https://origsvn.digium.com/svn/asterisk/team/mmichelson/threadpool@377351 65c4cc65-6c06-0410-ace0-fbb531ad65f3