test: used fixturesDir from fixtures modules

In test-fs-realpath-on-substed-drive, require common/fixtures
module and swapped the location of fixturesDir
from common to fixtures module.

PR-URL: https://github.com/nodejs/node/pull/16813


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>