test: use common/fixtures module in hash-seed test

Replace `common.fixturesDir` with `fixtures.path()` usage in
test/pummel/test-hash-seed.js.

PR-URL: nodejs#16823
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/pummel/test-hash-seed.js

@@ -1,20 +1,20 @@
 'use strict';
 
+// Check that spawn child doesn't create duplicated entries
+require('../common');
 const REPETITIONS = 2;
-
 const assert = require('assert');
-const common = require('../common');
-const cp = require('child_process');
-const path = require('path');
-const targetScript = path.resolve(common.fixturesDir, 'guess-hash-seed.js');
+const fixtures = require('../common/fixtures');
+const { spawnSync } = require('child_process');
+const targetScript = fixtures.path('guess-hash-seed.js');
 const seeds = [];
 
 for (let i = 0; i < REPETITIONS; ++i) {
-  const seed = cp.spawnSync(process.execPath, [targetScript],
-                            { encoding: 'utf8' }).stdout.trim();
+  const seed = spawnSync(process.execPath, [targetScript], {
+    encoding: 'utf8'
+  }).stdout.trim();
   seeds.push(seed);
 }
 
 console.log(`Seeds: ${seeds}`);
-const hasDuplicates = (new Set(seeds)).size !== seeds.length;
-assert.strictEqual(hasDuplicates, false);
+assert.strictEqual(new Set(seeds).size, seeds.length);