test: refactor test-require-extensions-main

* var => const
* assert test fixtures

PR-URL: https://github.com/nodejs/node/pull/9912
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/parallel/test-require-extensions-main.js

@@ -1,4 +1,10 @@
 'use strict';
-var common = require('../common');
+const assert = require('assert');
+const common = require('../common');
+const fixturesRequire = require(`${common.fixturesDir}/require-bin/bin/req.js`);
 
-require(common.fixturesDir + '/require-bin/bin/req.js');
+assert.strictEqual(
+    fixturesRequire,
+    '',
+    'test-require-extensions-main failed to import fixture requirements'
+);