Right now there aren't many unit tests in the Quicksilver code base. But with pull request #713, we added unit testing capabilities to the Quicksilver XCode project.
So, if you work on something that can be tested, please add some tests for it. Remember: Any tests are better than no tests.
The test classes should go in the corresponding group in XCode and the correct folder on disk, to keep them separate from the actual applications files. For example for tests of the QSCore.framework should go in Tests/QSCoreTests group and in the folder Tests/Tests-QSCore/.
They also need to be added to the correct test target. Currently, there are five different test targets: One for each of the four Quicksilver frameworks (QSCoreTests, QSEffectsTests, QSFoundationTests, and QSInterfaceTests). Put any tests directly related to each framework here. And one test target called QuicksilverTests. Put all other tests here. And don't add your test files to any of the normal (non-test-)targets.
For each test target there currently is a dummy test file. That needs to be there, because the target will fail if there are no test files. Once there are any real tests for a test target, the dummy test file can be removed.
To run the tests, just choose the target that contains your tests, and build it (with XCode 3) or (with Xcode 4) choose Test, not Build.
There is also a target called "Run All Tests", which doesn't contain any tests itself, but instead runs each of the five test targets on turn.