YAGNI does refer to features but I believe that the XP 'purists' would also say it extends to testing as well. I got this impression after reading Beck's Extreme Programming Explained. I will find the exact passage when I get the book back.
Do XP purists like Beck check for simple NullPointerExceptions in their APIs? I don't think so. They would see these checks as unnecessary. Of course these guys have been programming for years and years and they don't make these stupid mistakes. Expert programmers can leave these testing holes to reduce test-code coupling but most of us cannot afford to.
I think he's got this wrong. Tests (ideally) should cover all the possible failure points. This doesn't mean that we always acieve that, but - so far as I can tell - that's what we strive for. We don't just assume possible failures won't happen and not test for them...
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