test: run node-bench tools test sequentially

The test and its benchmark subprocesses allocate large histograms.
Run it sequentially to reduce memory pressure on constrained CI hosts.

Signed-off-by: Filip Skokan <panva.ip@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/66338
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>

test/sequential/test-benchmark-node-bench-tools.js

@@ -1,6 +1,9 @@
 // Flags: --no-warnings
 'use strict';
 
+// This test and its subprocesses allocate large, high-precision histograms.
+// Run sequentially to avoid exhausting memory on constrained CI hosts.
+
 require('../common');
 const assert = require('assert');
 const { spawnSync } = require('child_process');