test: use fixtures module over fixturesDir

clean up code by using fixtures helper module
instead of fixturesDir directly.

PR-URL: https://github.com/nodejs/node/pull/15847


Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>