test: fixup parallel/test-async-wrap-post-did-throw.js

* change var to let
* replace equal with strictEqual

PR-URL: https://github.com/nodejs/node/pull/8625


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>