test: use common.fixtures in tls test

PR-URL: #15965
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

Author: Ben Michel

test/parallel/test-tls-max-send-fragment.js

@@ -1,19 +1,20 @@
 'use strict';
 const common = require('../common');
+const fixtures = require('../common/fixtures');
+
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
 const assert = require('assert');
 const tls = require('tls');
-const fs = require('fs');
 
 const buf = Buffer.allocUnsafe(10000);
 let received = 0;
 const maxChunk = 768;
 
 const server = tls.createServer({
-  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')
 }, function(c) {
   // Lower and upper limits
   assert(!c.setMaxSendFragment(511));