samples: posix: eventfd: Add harness in sample.yaml verifing the output
samples: posix: eventfd: Add harness in sample.yaml verifing the output

The sample does not provide a way to verify its operation
and causes fails on nrf platforms. Added regex checks in sample.yaml
so the sample can be verified.

Signed-off-by: Maciej Perkowski <cb34e965b6a5a3425eafdbd0feb7e469ae1417ea@nordicsemi.no>

galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr