18.5. What to Test
Having a test suite that fails to fail might not be a major problem, so long as your tests cover the most common ways in which your software will actually be used. The single most important practice here is to run your tests on real-world cases.
That is, if you're building software to handle particular datasets or data streams, test it using actual samples of that data. And make sure those samples are of a similar size to the data on which the software will eventually need to operate.
Play-testing (see Chapter 17) can also come in handy here. If you (or other prospective users) have prototyped the kind of code you expect to write, then you should test the kinds of activities that your exploratory code implements, and the kinds of errors that you made when writing that code. Better yet, just write your hypothetical code as a test suite, using one of the Test:: modules. Then, when you're ready to implement, your test suite will already be in place.
Testing the most likely uses of your software is essential, but it's also vital to write tests that examine both edge-cases (i.e., one parameter with an extreme or unusual value) and corner-cases (i.e., several parameters with an extreme or unusual value).
Good places to hunt for bad behaviour include: