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 in the path-makelong test.

PR-URL: https://github.com/nodejs/node/pull/15908


Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>