tests/sys_atomic_utils_unittests: check return values
tests/sys_atomic_utils_unittests: check return values

Update the test to the new API and also verify correct return values.

jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,authmillenon/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kYc0o/RIOT,miri64/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,ant9000/RIOT,OTAkeys/RIOT,kYc0o/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OTAkeys/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,OTAkeys/RIOT,kaspar030/RIOT,kaspar030/RIOT,OTAkeys/RIOT,authmillenon/RIOT,miri64/RIOT