tests/unittests: tests-ztimer: add ztimer_is_set() regression test
tests/unittests: tests-ztimer: add ztimer_is_set() regression test

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