test: use common/fixtures module in hash-seed test

Replace `common.fixturesDir` with `fixtures.path()` usage in
test/pummel/test-hash-seed.js.

PR-URL: https://github.com/nodejs/node/pull/16823


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