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.
