test: fixtures in test-process-redirect-warnings

In test/parallel/test-process-redirect-warning.js replace usage of
common.fixture by common.fixturesDir

PR-URL: https://github.com/nodejs/node/pull/15917


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>