test: prepare test-hash-seed for CI

Reduce the time it takes to run test/pummel/test-hash-seed by switching
from spawnSync() to spawn(). On my computer, this reduces the runtime
from about 80 seconds to about 40 seconds. This test is not (yet) run
regularly on CI, but when it was run recently, it timed out.

test/pummel/test-hash-seed.js

@@ -2,19 +2,30 @@
 
 // Check that spawn child doesn't create duplicated entries
 require('../common');
+const Countdown = require('../common/countdown');
 const REPETITIONS = 2;
 const assert = require('assert');
 const fixtures = require('../common/fixtures');
-const { spawnSync } = require('child_process');
+const { spawn } = require('child_process');
 const targetScript = fixtures.path('guess-hash-seed.js');
 const seeds = [];
 
+const requiredCallback = () => {
+  console.log(`Seeds: ${seeds}`);
+  assert.strictEqual(new Set(seeds).size, seeds.length);
+  assert.strictEqual(seeds.length, REPETITIONS);
+};
+
+const countdown = new Countdown(REPETITIONS, requiredCallback);
+
 for (let i = 0; i < REPETITIONS; ++i) {
-  const seed = spawnSync(process.execPath, [targetScript], {
-    encoding: 'utf8'
-  }).stdout.trim();
-  seeds.push(seed);
-}
+  let result = '';
+  const subprocess = spawn(process.execPath, [targetScript]);
+  subprocess.stdout.setEncoding('utf8');
+  subprocess.stdout.on('data', (data) => { result += data; });
 
-console.log(`Seeds: ${seeds}`);
-assert.strictEqual(new Set(seeds).size, seeds.length);
+  subprocess.on('exit', () => {
+    seeds.push(result.trim());
+    countdown.dec();
+  });
+}