tests/periph_eeprom: use memcmp for cleared bytes
tests/periph_eeprom: use memcmp for cleared bytes

`strcmp` was used to check that the 4 bytes were empty but was not
testing what it should as an empty string is `0` bytes long.

The whole must be verified.

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