Depends on the API and how much testing you need. You want to test your code, not the API's availability or correctness.

But it can be as easy as using a fake http library and mocking the responses, or using a httptest server: https://onsi.github.io/gomega/#ghttp-testing-http-clients

If the API is complicated and you have to write your own fake server, that might not make sense for small projects.