Add a test for _writableState.needDrain.
PR-URL: https://github.com/nodejs/node/pull/8799
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Related: https://github.com/nodejs/node/issues/8686
