test: replaced fixturesDir with fixtures module

This was an assigned task at Node.js Interactive North America 2017.
This replaced the fixturesDir exported by the common module with
the fixturesDir on the common/fixtures module.

PR-URL: https://github.com/nodejs/node/pull/15881


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>