test: update fixturesDir to fixtures.readKey

PR-URL: #16016
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Refael Ackermann <refack@gmail.com>

test/parallel/test-https-agent-servername.js

@@ -5,12 +5,12 @@ if (!common.hasCrypto)
   common.skip('missing crypto');
 
 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')
 };