test: refactor test-listen-fd-ebadf

Replace var with const and assert.equal with assert.strictEqual.

PR-URL: nodejs#10034
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

test/parallel/test-listen-fd-ebadf.js

@@ -1,13 +1,13 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var net = require('net');
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
 
 net.createServer(common.fail).listen({fd: 2})
   .on('error', common.mustCall(onError));
 net.createServer(common.fail).listen({fd: 42})
   .on('error', common.mustCall(onError));
 
 function onError(ex) {
-  assert.equal(ex.code, 'EINVAL');
+  assert.strictEqual(ex.code, 'EINVAL');
 }