test: replace common.fixturesDir

In test/parallel/test-tls-junk-closes-server.js, replaced
common.fixtruesDir with usage of the common.fixtrues module.

PR-URL: #15834
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.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: Gireesh Punathil <gpunathi@in.ibm.com>

test/parallel/test-tls-junk-closes-server.js

@@ -1,16 +1,16 @@
 'use strict';
 const common = require('../common');
+const fixtures = require('../common/fixtures');
 
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
 const tls = require('tls');
-const fs = require('fs');
 const net = require('net');
 
 const options = {
-  key: fs.readFileSync(`${common.fixturesDir}/keys/agent2-key.pem`),
-  cert: fs.readFileSync(`${common.fixturesDir}/keys/agent2-cert.pem`)
+  key: fixtures.readKey('agent2-key.pem'),
+  cert: fixtures.readKey('agent2-cert.pem')
 };
 
 const server = tls.createServer(options, common.mustNotCall());