test: fix flaky test-pipe-unref

PR-URL: https://github.com/nodejs/node/pull/17950
Fixes: https://github.com/nodejs/node/issues/16875


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>