Code review comment for lp:~afrantzis/mir/fix-1477467-callback-before-wait-handle

> eh, still +1, but the bug seems to be covered well enough from the acceptance test,
> and the unit tests and the addition to WaitHandle seem less valuable

I added the unit tests because it was not easy to test the fix for the buffer_unavailable() path in the acceptance tests. Since I added the buffer_unavailable unit test I also added the buffer_available() test for completeness.