tests/README.md: add paragraph explaining expect()
tests/README.md: add paragraph explaining expect()

miri64/RIOT,kaspar030/RIOT,yogo1212/RIOT,basilfx/RIOT,smlng/RIOT,basilfx/RIOT,OlegHahm/RIOT,basilfx/RIOT,authmillenon/RIOT,miri64/RIOT,yogo1212/RIOT,yogo1212/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,basilfx/RIOT,smlng/RIOT,authmillenon/RIOT,ant9000/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,kaspar030/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,smlng/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,basilfx/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT,smlng/RIOT,OTAkeys/RIOT,ant9000/RIOT,yogo1212/RIOT,miri64/RIOT,RIOT-OS/RIOT,smlng/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,jasonatran/RIOT,yogo1212/RIOT,ant9000/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,authmillenon/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kYc0o/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT