test: use fixtures module

Replace usage of common.fixturesDir with fixtures module in
test-https-agent-disable-session-reuse.js.

PR-URL: nodejs/node#15901
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

test/parallel/test-https-agent-disable-session-reuse.js

@@ -1,17 +1,19 @@
 'use strict';
 const common = require('../common');
+
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+const fixtures = require('../common/fixtures');
+
 const assert = require('assert');
 const https = require('https');
-const fs = require('fs');
 
 const TOTAL_REQS = 2;
 
 const options = {
-  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')
 };
 
 const clientSessions = [];