test: make use of common/fixtures.fixturesDir

The common/fixtures module provides convenience methods for working with
files in the test/fixtures directory. Make use of the `fixturesDir`
value from module rather than value from common.

- Replace `common.fixtures` with `fixtures.fixturesDir`

PR-URL: https://github.com/nodejs/node/pull/15815
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/pummel/test-crypto-timing-safe-equal-benchmarks.js

@@ -6,11 +6,12 @@ if (!common.hasCrypto)
 if (!common.enoughTestMem)
   common.skip('memory-intensive test');
 
+const fixtures = require('../common/fixtures');
 const assert = require('assert');
 const crypto = require('crypto');
 
 const BENCHMARK_FUNC_PATH =
-  `${common.fixturesDir}/crypto-timing-safe-equal-benchmark-func`;
+  `${fixtures.fixturesDir}/crypto-timing-safe-equal-benchmark-func`;
 function runOneBenchmark(...args) {
   const benchmarkFunc = require(BENCHMARK_FUNC_PATH);
   const result = benchmarkFunc(...args);