test: replace fixturesDir with common.fixtures

PR-URL: #15810
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

test/parallel/test-tls-key-mismatch.js

@@ -4,15 +4,16 @@ 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 errorMessageRegex =
   /^Error: error:0B080074:x509 certificate routines:X509_check_private_key:key values mismatch$/;
 
 const options = {
-  key: fs.readFileSync(`${common.fixturesDir}/keys/agent1-key.pem`),
-  cert: fs.readFileSync(`${common.fixturesDir}/keys/agent2-cert.pem`)
+  key: fixtures.readKey('agent1-key.pem'),
+  cert: fixtures.readKey('agent2-cert.pem')
 };
 
 assert.throws(function() {