XP Unit Tests are usually implemented at
the class level (in OO languages) and test the public interface. For every class in the system, a test class exists.
Popular test frameworks are available usually for free (such as JUnit, CppUnit, CppUnitLite, NUnit). These are simple
tools that help the developer to organize and run unit tests. These tests drive the programmer development and tell the
programmer that the code works as the programmer expects.