test: replace fixturesDir with fixtures.readKey

PR-URL: #15948
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

test/parallel/test-https-agent-secure-protocol.js

@@ -8,12 +8,12 @@ if (!common.hasCrypto) {
 }
 
 const https = require('https');
-const fs = require('fs');
+const fixtures = require('../common/fixtures');
 
 const options = {
-  key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
-  cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem'),
-  ca: fs.readFileSync(common.fixturesDir + '/keys/ca1-cert.pem')
+  key: fixtures.readKey('agent1-key.pem'),
+  cert: fixtures.readKey('agent1-cert.pem'),
+  ca: fixtures.readKey('ca1-cert.pem')
 };
 
 const server = https.Server(options, function(req, res) {