What is end-to-end testing for GTM workflows?
End-to-end testing is running a workflow from the first trigger to the final customer-facing result and confirming every expected outcome happened, not just that each step fired.
For a sign-up path, that means submitting the form on a phone and a laptop, confirming the CRM record exists, confirming the right person was alerted, and confirming the customer received their message.
Run it on a schedule and before every launch. Automate it where you can, and have a second person run it where you can't, since the person who built a workflow is the least likely to notice what is missing.