test: use Promise.all() in test-hash-seed

We have several tests where a number of asynchronous processes need to
finish before some checks happen. These are done in a number of ways,
including (as here) using our Countdown testing module. I think
Promise.all() may be the idiomatic and ergonomic way to go for a lot of
these tests. Using this one to get feedback on the idea.

PR-URL: #32273
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>

test/pummel/test-hash-seed.js

@@ -1,31 +1,26 @@
 'use strict';
 
 // Check that spawn child doesn't create duplicated entries
-require('../common');
-const Countdown = require('../common/countdown');
-const REPETITIONS = 2;
+const common = require('../common');
+const kRepetitions = 2;
 const assert = require('assert');
 const fixtures = require('../common/fixtures');
-const { spawn } = require('child_process');
+const { promisify, debuglog } = require('util');
+const debug = debuglog('test');
+
+const { execFile } = require('child_process');
+const execFilePromise = promisify(execFile);
 const targetScript = fixtures.path('guess-hash-seed.js');
-const seeds = [];
 
-const requiredCallback = () => {
-  console.log(`Seeds: ${seeds}`);
+const requiredCallback = common.mustCall((results) => {
+  const seeds = results.map((val) => val.stdout.trim());
+  debug(`Seeds: ${seeds}`);
   assert.strictEqual(new Set(seeds).size, seeds.length);
-  assert.strictEqual(seeds.length, REPETITIONS);
-};
-
-const countdown = new Countdown(REPETITIONS, requiredCallback);
+  assert.strictEqual(seeds.length, kRepetitions);
+});
 
-for (let i = 0; i < REPETITIONS; ++i) {
-  let result = '';
-  const subprocess = spawn(process.execPath, [targetScript]);
-  subprocess.stdout.setEncoding('utf8');
-  subprocess.stdout.on('data', (data) => { result += data; });
+const generateSeed = () => execFilePromise(process.execPath, [targetScript]);
+const subprocesses = [...new Array(kRepetitions)].map(generateSeed);
 
-  subprocess.on('exit', () => {
-    seeds.push(result.trim());
-    countdown.dec();
-  });
-}
+Promise.all(subprocesses)
+  .then(requiredCallback);