Network Link Conditioner
This is a useful tool for seeing how your app works under different network settings.
A side effect of using Network Link Conditioner is that you can also identify when a test is mocking out the network by using a custom NSURLProtocol. Because those tests will also be affected by the network link conditioner. This is part of why if your unit test makes a network call, it’s not a unit test. Even touching the URL loading subsystem is making a network call.
By the way, NSHipster also has an excellent article on NSURLProtocol, because it is useful for mocking network requests for integration-style tests and the like.