test: change common.fixturesDir to fixtures.path

Use common.fixtures module instead of common.fixturesDir based on task
at Nodejs code and learn at Node.js Interactive 2017 in Vancouver.

PR-URL: https://github.com/nodejs/node/pull/15860


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>