tests/pkg_fff: use MCU instead of CPU_FAM for ESP32x SoC
tests/pkg_fff: use MCU instead of CPU_FAM for ESP32x SoC

To be able to compile the test for different ESP32x variants without defining each ESP32x variant, MCU has to be used instead of CPU_FAM.
kYc0o/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,ant9000/RIOT,kYc0o/RIOT,RIOT-OS/RIOT