What should be tested is that the function does what is promised to the caller. Which other function it uses to accomplish this task is a detail, which should generally not be part of this promise (encapsulation). So either the effects of the function must be returned by the function, or it is a side-effect, which must then be observable in some other way.
Example: If a function is supposed to create new user, don't check that it calls some internal persistence or communication layer with some User info. Instead list the users in system before and after, check that the correct one was added. Try to make an action as this new user.
This forces you to expose (a view of) internal state at the API surface, which makes the system more observable, usually very beneficial for debugging and fillings gaps in API.
Also, many of the assertions that are often put into unit-tests (especially at stub/mock boundaries) are better formulated as invariants, checked in the code itself. Design-by-contracts style pre/post-conditions is a sound and practical way of doing this. When this is done well, you get the localization part of low-level unit testing even when running high-level tests. Plus much better coverage, since these things are always checked (even in prod), not just in a couple of unit tests. And it is more natural when refactoring internal functions to update pre/post-conditions, since they are right there in the code. When a function disappears they also do.
I don't like the term "integration" tests though, as they hint at interactions between systems being the important thing to test. Integration between services / subsystems are just as much a detail as internal function calls. If using the real system during test is too complicated or slow, maybe it should be simplified or made faster? Only when that is not feasible do I build a mock.
Also, many of the assertions that are often put into unit-tests (especially at stub/mock boundaries) are better formulated as invariants, checked in the code itself. Design-by-contracts style pre/post-conditions is a sound and practical way of doing this. When this is done well, you get the localization part of low-level unit testing even when running high-level tests. Plus much better coverage, since these things are always checked (even in prod), not just in a couple of unit tests. And it is more natural when refactoring internal functions to update pre/post-conditions, since they are right there in the code. When a function disappears they also do.
I don't like the term "integration" tests though, as they hint at interactions between systems being the important thing to test. Integration between services / subsystems are just as much a detail as internal function calls. If using the real system during test is too complicated or slow, maybe it should be simplified or made faster? Only when that is not feasible do I build a mock.