test: add test-zlib-flush-drain

This test assures that if flush is called while the zlib object needs
to be drained that it will defer the callback until after the drain.

PR-URL: https://github.com/nodejs/node/pull/3534


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>