test: make use of common/fixtures.fixturesDir

The common/fixtures module provides convenience methods for working with
files in the test/fixtures directory. Make use of the `fixturesDir`
value from module rather than value from common.

- Replace `common.fixtures` with `fixtures.fixturesDir`

PR-URL: https://github.com/nodejs/node/pull/15815


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