tests/Kconfig: add samd21-xpro to whitelist
tests/Kconfig: add samd21-xpro to whitelist

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