Postman is the SAP Cloud Integration developer's favorite tool.
But you can start the development without it; you need an input message and upload it to Figaf.
Then you can start the development process and test what it will be creating. Once you are done with the development, you have a test case the next developer can use.
You would also be able to record the message from the first time the business sends a real document and then use it for your development process.
The tool can support other protocols like JMS or ProcessDirct similarly. That way, you don't need the HTTP to x bridge in your iflow.