test: replace fixturesDir with fixtures method

PR-URL: https://github.com/nodejs/node/pull/15894
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-https-byteswritten.js

@@ -21,16 +21,16 @@
 
 'use strict';
 const common = require('../common');
+const fixtures = require('../common/fixtures');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
 const assert = require('assert');
-const fs = require('fs');
 const https = require('https');
 
 const options = {
-  key: fs.readFileSync(`${common.fixturesDir}/keys/agent1-key.pem`),
-  cert: fs.readFileSync(`${common.fixturesDir}/keys/agent1-cert.pem`)
+  key: fixtures.readKey('agent1-key.pem'),
+  cert: fixtures.readKey('agent1-cert.pem')
 };
 
 const body = 'hello world\n';