Refactored cross sphere line tracking and added a unittest to verify correct operation

cmake/configs/posix_sitl_default.cmake

src/modules/mc_pos_control/mc_pos_control_main.cpp

...odules/mc_pos_control/mc_pos_control_tests/CMakeLists.txt

...pos_control/mc_pos_control_tests/mc_pos_control_tests.cpp

src/systemcmds/tests/tests_main.c

src/systemcmds/tests/tests_main.h