tests/periph_flashpage_unittest: lower FLASHPAGE_SIZE for int overflow
tests/periph_flashpage_unittest: lower FLASHPAGE_SIZE for int overflow

On AVR_8 512*256 causes an int overflow.

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