How to write automated tests when using cloud APIs?

I'm adding to an open source project that uses in this case some Azure cloud functionality, but the same general problem is applicable for any cloud API. I want to write tests for my code, but the results of the test are reliant on something having happened in the cloud service I'm using, and in order for this to happen, I need to supply credentials to the cloud service. In a private project, I could certainly just add my cloud credentials to the testing environment, but for public/open source projects, I can't do this. I can test locally easily enough, but this project uses CI (as do many OSS projects), so this can't really be done.

One approach seems to be using mock or something similar, but that doesn't actually seem to test that things are happening as they should be, and strikes me as a mostly pointless method to achieve 100% coverage.

Are there any 'virtual test cloud' environments that can be spun up to create an identical interface to the cloud service in question, but only for testing? How do these deal with side effects (the code in question creates a DNS entry, and ideally would test for the actual existence of a DNS entry using the system's resolver rather than another cloud call)?

How do people do this kind of testing?