test: fix flaky test-benchmark-fs

Some benchmarks may return 0 operations with the new very short duration
provided by the test program. Set environment variable to allow that.

PR-URL: https://github.com/nodejs/node/pull/17885


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>