test: replace common.fixtureDir with fixtures

PR-URL: https://github.com/nodejs/node/pull/15816


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>