test: use fixtures.readKey in https-agent test

PR-URL: #15913
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-https-agent-create-connection.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const common = require('../common');
+const fixtures = require('../common/fixtures');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
@@ -9,11 +10,9 @@ const https = require('https');
 
 const agent = new https.Agent();
 
-const fs = require('fs');
-
 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 expectedHeader = /^HTTP\/1\.1 200 OK/;