While testing REST APIs for one of my projects, I found REST Assured. It was perfect, as it took care of low level HTTP calls under the hood, and provided a high-level, easy to use framework to write tests. Not only REST Assured works really well to test webMethods flow services, you can also run these tests as part of Continuous Integration process through Gradle. You will need following things on your machine.
If a specific functionality is not available out of box from the integration server, you often need to use external jars or java libraries in IS. e.g. If you want to generate a PDF document or excel spreadsheet on your IS, you need to use your preferred libraries like IText or Apache POI. It is not recommended to place your custom jars in the IntegrationServer/lib/jars folder. But Integration Server provides you couple of places to place these jars depending on how you want them to be loaded.