Don't waste time testing cron-based hook points using the actual cron. Save time as follows:
While I'm still coding, I comment AfterCronJob trigger and temporarily replace it with AdminAreaPage. At the very top of the function I put a condition that immediatlly exits the script if the query string is missing simulate_cron parameter.
In other words I can browse WHMCS backend as normal. Whenever I want to simulate the cron I simply need to add ?simulate_cron=blabla in the query string on any page I am (eg. yourwhmcs.example.com/clients.php?simulate_cron=1). This way you can test your script without waiting for real cron job. Moreover if there's an error you can see that on screen.
When you're ready to go live simply remove AdminAreaPage trigger and remove comment for AfterCronJob. Alternatively you can use an approach like this one basically a button you can press to run the cron job on click.