test: use fixtures module in test-https-pfx

PR-URL: nodejs/node#15895
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-https-pfx.js

@@ -21,14 +21,16 @@
 
 'use strict';
 const common = require('../common');
+
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+const fixtures = require('../common/fixtures');
+
 const assert = require('assert');
-const fs = require('fs');
 const https = require('https');
 
-const pfx = fs.readFileSync(`${common.fixturesDir}/test_cert.pfx`);
+const pfx = fixtures.readSync('test_cert.pfx');
 
 const options = {
   host: '127.0.0.1',