By-passing function modules in aunits

Walks through registering a function module as a test double with CL_FUNCTION_TEST_ENVIRONMENT inside an ABAP Unit test class: class_setup names the function module to replace, an input configuration matches on the importing parameters, and an output configuration sets what the double returns before the calling class is asserted against it. The Function Test Double Framework is presented as ABAP Unit's way of testing code that calls a function module — procedural, without a class's native test-double support — without triggering the database, RFC or BAPI calls the real function module would make.