tests can now be built as part of FSFW

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.

tests/src/fsfw_tests/unit/timemanager/CMakeLists.txt

@@ -1,3 +1,3 @@
-target_sources(${TARGET_NAME} PRIVATE
+target_sources(${FSFW_TEST_TGT} PRIVATE
 	TestCountdown.cpp
 )