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 <andrzej.glabek@nordicsemi.no>