Mountebank is the best testing tool you've never heard of. It is a free, open source and cross platform tool for multi-protocol test doubles over the wire. When your code calls HTTP(s) endpoints (or TCP or SMTP and more), Mountebank can stand in for the actual service, allowing you to effectively recreate scenarios that are difficult or even impossible when using an actual running service. These "imposters" are created at any point in your pipeline by sending a request to the locally executing Mountebank service that defines the parameters, predicates and responses to return. This means behaviors can be scripted at runtime and be based on any business logic you define. When your tests are done, simply tear down the imposters. It's OK, because your service fakes are defined as JSON and can be source controlled and stood up in milliseconds for the next run.

Don't spend hours or days standing up an API specifically for testing. Don't wait around for someone else to get an API stood up so you can move ahead with your project. Fake it until you make it!