test: replace fixturesDir with fixtures module

PR-URL: nodejs#15848
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-tls-disable-renegotiation.js

@@ -1,7 +1,7 @@
 'use strict';
 const common = require('../common');
 const assert = require('assert');
-const fs = require('fs');
+const fixtures = require('../common/fixtures');
 
 // Tests that calling disableRenegotiation on a TLSSocket stops renegotiation.
 
@@ -11,8 +11,8 @@ if (!common.hasCrypto)
 const tls = require('tls');
 
 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 server = tls.Server(options, common.mustCall((socket) => {