Mock code for external modules

[PatrickKa]

Description

Mocking the code for a module should work as follows:

• The code for the mock should be in <Module>Mock.[chi]pp.
• Whether to use the mock or the real code is decided with CMake by adding <Module>.cpp or <Module>Mock.cpp to the target sources.
• Similar to the non-virtual interface (NVI) design pattern, the public functions that need to be mocked and are declared in <Module>.hpp, forward their calls to functions of the same name but with prefix Do, _, Mock or something like that. I'll call them "do functions" from now on.
• These do functions are actually function pointers. For each one, a set function must be declared in <Module>Mock.hpp and defined in <Module>Mock.cpp. As the name suggests, the set functions are used to chose where the function pointers point to. This means that you can choose and change the implementation of the mocked function at runtime.
• All function pointers must be initialized with a default function that does "nothing". This way, tests using <Module>Mock.cpp instead of <Module>.cpp will at least compile without any further changes.

To do

• Mock FRAM code #207
• Mock RF code #208
• Mock file system code #209
• Mock EDU code #210