Allow me to say again that this is a really, really good test plan that includes both positive and negative tests (and also includes regression tests to see that you didn't break anything that already was working!)
In general, running tests like these (ie, not the automated software test suite but a more directed QA form of testing) is an important part of coding, because it not only reassures you that what you've done works, it assures you that you haven't broken anything. (And sometimes you can discover a new bug in the existing code and fix it along with whatever you're working on!) Having a system for checking all the various variations on what users are likely to do to your patch when it gets live really forces you to think about what you're changing and what it might affect, and this is a great example of covering all the bases.
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In general, running tests like these (ie, not the automated software test suite but a more directed QA form of testing) is an important part of coding, because it not only reassures you that what you've done works, it assures you that you haven't broken anything. (And sometimes you can discover a new bug in the existing code and fix it along with whatever you're working on!) Having a system for checking all the various variations on what users are likely to do to your patch when it gets live really forces you to think about what you're changing and what it might affect, and this is a great example of covering all the bases.