Leaves in this context would be classes that have no dependencies.
If you need to create an object, can you pass the name of the class in? Or can the object be created elsewhere and passed in fresh? If you're making a call to a remote service (even your local DB) are you being passed a proxy object?
All of these references can then be provided as a test double or test spy, so long as they are strict about the interface they provide/expect, and you can exhaustively cover whatever internal edge cases you need with unit tests.
Don't _forget_ the integration tests, but my personal opinion is that it usually suffices to have one "success" and one "error" integration test to cover the whole stack, and then rely on unit tests to be more exhaustive about handling the possible error cases.
If you need to create an object, can you pass the name of the class in? Or can the object be created elsewhere and passed in fresh? If you're making a call to a remote service (even your local DB) are you being passed a proxy object?
All of these references can then be provided as a test double or test spy, so long as they are strict about the interface they provide/expect, and you can exhaustively cover whatever internal edge cases you need with unit tests.
Don't _forget_ the integration tests, but my personal opinion is that it usually suffices to have one "success" and one "error" integration test to cover the whole stack, and then rely on unit tests to be more exhaustive about handling the possible error cases.