Comment by randomdata
12 days ago
Keep in mind that there are different kinds of testing. What Beck called unit tests and integration tests.
Unit tests are really for purposes of documentation. They show future programmers the intent and usage of a function/interface so that others can figure out what you were trying to do. Mocking is fine here as future programmers are not looking to learn about the message system here. They will refer to the message system's own documentation when they need to know something about the message system.
Integration tests are for the more classical view on testing. Beck suggested this is done by another team using different tools (e.g. UI control software), but regardless of specifics it is done as a whole system. This is where you would look for such failure points.
Unit tests as a form of example code based documentation is where I could see unit tests complimenting documentation, yes.
However, depending on the industry, code coverage is a valuable tool to gauge the maturity of the software baseline and burning down software execution risk. One example of this is Airworthiness or Safety Critical Code.
Of course, there is no single code coverage metric. Code covered by unit tests does not count towards code covered by integration tests. They are completely separate systems. And, at least in Beck's opinion, should be carried out by completely different teams.