test: change var to const in test-tls-key-mismatch.js

PR-URL: #9897
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

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

@@ -1,15 +1,15 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
+const common = require('../common');
 
 if (!common.hasCrypto) {
   common.skip('missing crypto');
   return;
 }
-var tls = require('tls');
-var fs = require('fs');
+const assert = require('assert');
+const tls = require('tls');
+const fs = require('fs');
 
-var options = {
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem')
 };