test: fix linting for test-tls-add-ca-cert.js

Fix lint error introduced in ea72331af

PR-URL: https://github.com/nodejs/node/pull/10771
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/internet/test-tls-add-ca-cert.js

@@ -22,7 +22,7 @@ function loadPEM(n) {
 
 const caCert = loadPEM('ca1-cert');
 
-var opts = {
+const opts = {
   host: 'www.nodejs.org',
   port: 443,
   rejectUnauthorized: true