test: refactor test-internal-modules

* var -> const
* add RegExp to assert.throws() to check error message

PR-URL: https://github.com/nodejs/node/pull/10016
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-internal-modules.js

@@ -1,11 +1,11 @@
 'use strict';
-var common = require('../common');
-var path = require('path');
-var assert = require('assert');
+const common = require('../common');
+const path = require('path');
+const assert = require('assert');
 
 assert.throws(function() {
   require('internal/freelist');
-});
+}, /^Error: Cannot find module 'internal\/freelist'$/);
 
 assert.strictEqual(
   require(path.join(common.fixturesDir, 'internal-modules')),