The purpose of the Testability Class is
to capture the design for the test-specific functionality required to facilitate testing. This test-specific
functionality should be incorporated in the software design model so that it can be factored into the complete software
design. There are various types of test-specific behavior, two of which are:
"Stubs" for design classes that you need to simulate (typically because a complete version is not yet available).
Specialized interfaces or output, that provide the visibility or control necessary to conduct testing.