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>

samples/drivers/spi_flash_at45/sample.yaml

@@ -1,6 +1,26 @@
 sample:
   name: SPI Flash AT45 Sample
 tests:
+  sample.drivers.spi.flash.at45.build:
+    tags: spi flash
+    depends_on: spi
+    filter: dt_compat_enabled("atmel,at45")
+    build_only: true
   sample.drivers.spi.flash.at45:
     tags: spi flash
-    platform_whitelist: nrf9160dk_nrf9160
+    depends_on: spi
+    filter: dt_compat_enabled("atmel,at45")
+    harness: console
+    harness_config:
+      type: multi_line
+      ordered: true
+      regex:
+        - "Using \\w+, chip size: \\d+ bytes \\(page: \\d+\\)"
+        - "Reading the first byte of the test region ... OK"
+        - "Preparing test content starting with 0x[0-9a-f]{2}."
+        - "Writing the first half of the test region... OK"
+        - "Writing the second half of the test region... OK"
+        - "Reading the whole test region... OK"
+        - "Checking the read content... OK"
+        - "Putting the flash device into low power state... OK"
+      fixture: atmel_at45