test: refactor to eliminate __defineGetter__

In preparation for a lint rule to flag `__defineGetter__`, refactor the
one remaining instance in the code base.

PR-URL: https://github.com/nodejs/node/pull/6774


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