This PR refactores the tests so they are built as part of the FSFW.
This is done by adding Catch2 with the FetchContent directive.
A future implementation might also use a system installation of Catch2
by first checking whether Catch2 can already be found as a package
The custom configuration folder testcfg was moved from the user folder
to the actual unittest folder.
The tests can be built by setting the CMake FSFW_BUILD_UNITTESTS option
to TRUE/ON. They are built with the static library and dropped inside
the build folders fsfw directory.
1. Further reduces the amount of code the user needs to copy and paste
2. Makes FSFW tests more accessible. This can be used to simplify moving mission unit tests
to the FSFW
3. A lot of include improvements
