test: replace common.fixturesDir with fixtures

PR-URL: https://github.com/nodejs/node/pull/15857
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/parallel/test-tls-client-resume.js

@@ -30,11 +30,11 @@ if (!common.hasCrypto)
 
 const assert = require('assert');
 const tls = require('tls');
-const fs = require('fs');
+const fixtures = require('../common/fixtures');
 
 const options = {
-  key: fs.readFileSync(`${common.fixturesDir}/keys/agent2-key.pem`),
+  key: fixtures.readKey('agent2-key.pem'),
-  cert: fs.readFileSync(`${common.fixturesDir}/keys/agent2-cert.pem`)
+  cert: fixtures.readKey('agent2-cert.pem')
 };
 
 // create server