Ah! coincidence. My colleague sam just noticed this, and so on our next Moodle Monday here at the OU, we will be working to try to get all the tests passing. See MDL-24905.
Information about the Moodle unit testing syste is on Development:Unit_tests. The failed test probably just mean that no one has bothered to run them for a while, and the code being tested has changed in a way that, while not buggy, breaks the tests.