The test directory contains our unit tests. You can run them at /test/index.php Running the tests will generate some content in your network, so it's not advisable on anything other than a test network. You can look at some of the files in /test to get an idea of how to create your own unit test for any additions to the code you make.
I've made a test that the test runner can find, but I have a syntax error or something that's giving me a White Screen of Doom. I don't see anything in my network's logs through /admin/index/log -- is there somewhere else I could see my test-related exceptions? Some other log source that I'm not seeing?