samples/drivers/spi_flash_at45: Improve test case descriptions
samples/drivers/spi_flash_at45: Improve test case descriptions

Instead of whitelist, use a filter that checks if a required
"atmel,at45" compatible node is present.
Add a build-only test case to be used by CI and provide a harness
configuration so that execution of the sample can be also verified.

Signed-off-by: Andrzej Głąbek <6788b8d556a8a447102338fc996c42180ecff44e@nordicsemi.no>

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