I have to disagree with you. This is a perfectly reasonable request. In fact, this is considered a good practice for testing complex software.

There are lots of reasons for doing things like this:

1. You may not have control over function y
2. It might do something that you don't want to do during testing (such as submit a credit card order...)
3. It might take a long time to run.
4. It might be hard to reproduce certain kinds of output.
5. It might not be written yet, or it might have bugs of its own (simulating part of the software allows you to develop/test multiple interdependent pieces of code simultaneously).