test: fix flaky test-benchmark-buffer

Allow zero operations for short buffer benchmark tests.

PR-URL: https://github.com/nodejs/node/pull/16296
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/sequential/test-benchmark-buffer.js

@@ -20,5 +20,5 @@ runBenchmark('buffers',
                'source=array',
                'type=',
                'withTotalLength=0'
-
-             ]);
+             ],
+             { NODEJS_BENCHMARK_ZERO_ALLOWED: 1 });