test: replace fixtureDir with fixtures module

This commit replaces the usage of common.fixturesDir with
fixtures.path.

PR-URL: https://github.com/nodejs/node/pull/15823


Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>