test: changed vars to const in test-net-better-error-messages-listen-path.js

* `var` to `const`
* use `assert.strictEqual`

PR-URL: nodejs#9905
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: anoff

test/parallel/test-net-better-error-messages-listen-path.js

@@ -1,10 +1,10 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var net = require('net');
-var fp = '/blah/fadfa';
-var server = net.createServer(common.fail);
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+const fp = '/blah/fadfa';
+const server = net.createServer(common.fail);
 server.listen(fp, common.fail);
 server.on('error', common.mustCall(function(e) {
-  assert.equal(e.address, fp);
+  assert.strictEqual(e.address, fp);
 }));