Test Classes
Test classes are local classes in ABAP programs, such as executable programs, class pools, function groups, module pools, and subroutine pools. Exceptions are type groups and interface pools.
Test classes are defined using the FOR TESTING addition to the CLASS statement.
You can define test attributes for test classes that contain test methods.

You can create global test class pools for reusing tests and services. They must be abstract and must only be used in local test classes. All instance methods of a test class pool are test methods by default.