test: refactor test-debug-args

* indexOf() -> includes()
* var -> const

PR-URL: nodejs#9833
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: Trott

test/parallel/test-debug-args.js

@@ -2,6 +2,6 @@
 // Flags: --debug-code
 
 require('../common');
-var assert = require('assert');
+const assert = require('assert');
 
-assert.notEqual(process.execArgv.indexOf('--debug-code'), -1);
+assert(process.execArgv.includes('--debug-code'));