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: #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>

test/parallel/test-benchmark-fs.js

@@ -16,4 +16,4 @@ runBenchmark('fs', [
   'statSyncType=fstatSync',
   'encodingType=buf',
   'filesize=1024'
-], { NODE_TMPDIR: common.tmpDir });
+], { NODE_TMPDIR: common.tmpDir, NODEJS_BENCHMARK_ZERO_ALLOWED: 1 });