test: use fixtures in tls-friendly-error-message

PR-URL: nodejs/node#15905
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-tls-friendly-error-message.js

@@ -24,12 +24,12 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+const fixtures = require('../common/fixtures');
 const assert = require('assert');
 const tls = require('tls');
-const fs = require('fs');
 
-const key = fs.readFileSync(`${common.fixturesDir}/keys/agent1-key.pem`);
-const cert = fs.readFileSync(`${common.fixturesDir}/keys/agent1-cert.pem`);
+const key = fixtures.readKey('agent1-key.pem');
+const cert = fixtures.readKey('agent1-cert.pem');
 
 tls.createServer({ key: key, cert: cert }, common.mustCall(function(conn) {
   conn.end();