test: use of fixtures in test-pipe-head

Replace usage of common.fixturesDir by using common.fixtures
module in test/parallel/test-pipe-head

PR-URL: https://github.com/nodejs/node/pull/15868


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>