tests/periph_eeprom: take EEPROM_CLEAR_BYTE into account
tests/periph_eeprom: take EEPROM_CLEAR_BYTE into account

After being cleared, the memory can be `0xFF` for `atmega` platforms for
example.

Fix the test to take EEPROM_CLEAR_BYTE into account.

rfuentess/RIOT,ant9000/RIOT,BytesGalore/RIOT,miri64/RIOT,kYc0o/RIOT,smlng/RIOT,ant9000/RIOT,authmillenon/RIOT,josephnoir/RIOT,smlng/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,aeneby/RIOT,A-Paul/RIOT,kYc0o/RIOT,basilfx/RIOT,mfrey/RIOT,basilfx/RIOT,miri64/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,ant9000/RIOT,kaspar030/RIOT,lazytech-org/RIOT,cladmi/RIOT,mfrey/RIOT,smlng/RIOT,yogo1212/RIOT,josephnoir/RIOT,josephnoir/RIOT,mtausig/RIOT,mtausig/RIOT,lazytech-org/RIOT,BytesGalore/RIOT,smlng/RIOT,jasonatran/RIOT,A-Paul/RIOT,OlegHahm/RIOT,BytesGalore/RIOT,jasonatran/RIOT,authmillenon/RIOT,rfuentess/RIOT,yogo1212/RIOT,toonst/RIOT,mtausig/RIOT,yogo1212/RIOT,authmillenon/RIOT,kYc0o/RIOT,toonst/RIOT,jasonatran/RIOT,kaspar030/RIOT,yogo1212/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,lazytech-org/RIOT,miri64/RIOT,kYc0o/RIOT,lazytech-org/RIOT,aeneby/RIOT,yogo1212/RIOT,ant9000/RIOT,toonst/RIOT,basilfx/RIOT,x3ro/RIOT,lazytech-org/RIOT,basilfx/RIOT,toonst/RIOT,yogo1212/RIOT,x3ro/RIOT,aeneby/RIOT,aeneby/RIOT,basilfx/RIOT,RIOT-OS/RIOT,BytesGalore/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,mfrey/RIOT,OTAkeys/RIOT,kaspar030/RIOT,mfrey/RIOT,jasonatran/RIOT,mfrey/RIOT,x3ro/RIOT,rfuentess/RIOT,x3ro/RIOT,jasonatran/RIOT,cladmi/RIOT,miri64/RIOT,BytesGalore/RIOT,mtausig/RIOT,x3ro/RIOT,authmillenon/RIOT,authmillenon/RIOT,mtausig/RIOT,OlegHahm/RIOT,A-Paul/RIOT,cladmi/RIOT,rfuentess/RIOT,miri64/RIOT,OTAkeys/RIOT,A-Paul/RIOT,cladmi/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,OTAkeys/RIOT,josephnoir/RIOT,aeneby/RIOT,authmillenon/RIOT,cladmi/RIOT,smlng/RIOT,kaspar030/RIOT,toonst/RIOT,A-Paul/RIOT