There is a doxygen documentation about the testing in MITK:

http://docs.mitk.org/2014.03/GeneralTests.html

Since a while we are using CPPUNIT based test for new tests, however this is not yet documented, except of a short introduction by thomas kilgus:

http://mitk.org/images/5/5d/BugSquashingSeminars$CppUnitFrameworkSeminar.pdf