Make `src/Classic/Solvers/GreenWakeFunction::testApply()` a unit test

The following discussion from !269 (merged) should be addressed:

@snuverink_j started a discussion:

It should be made into a unit test. I will open a new issue for it.

The preprocesser definition #ifdef ENABLE_WAKE_TESTS and the function testApply() in src/Classic/Solvers/GreenWakeFunction.cpp should be transformed into a unit test.

Edited Feb 04, 2020 by snuverink_j