test: use fixtures in test-tls-multi-key.js

PR-URL: #15844
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-tls-multi-key.js

@@ -3,18 +3,18 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+const fixtures = require('../common/fixtures');
 const assert = require('assert');
 const tls = require('tls');
-const fs = require('fs');
 
 const options = {
   key: [
-    fs.readFileSync(`${common.fixturesDir}/keys/ec-key.pem`),
-    fs.readFileSync(`${common.fixturesDir}/keys/agent1-key.pem`),
+    fixtures.readKey('ec-key.pem'),
+    fixtures.readKey('agent1-key.pem'),
   ],
   cert: [
-    fs.readFileSync(`${common.fixturesDir}/keys/agent1-cert.pem`),
-    fs.readFileSync(`${common.fixturesDir}/keys/ec-cert.pem`)
+    fixtures.readKey('agent1-cert.pem'),
+    fixtures.readKey('ec-cert.pem'),
   ]
 };