samples: soc_flash_nrf: Print finished message

Print a finished message, so that it is clear that this in the end
of the sample. This makes it certain that we didn't crash without
output on the last test.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>

samples/drivers/soc_flash_nrf/README.rst

@@ -131,3 +131,5 @@ Sample Output
 
    Test 8: Write block size API
       write-block-size = 1
+
+   Finished!

samples/drivers/soc_flash_nrf/sample.yaml

@@ -29,3 +29,4 @@ tests:
         - "Data read matches data written. Good!"
         - "SoC flash consists of \\d+ pages."
         - "write-block-size = 1"
+        - "Finished!"

samples/drivers/soc_flash_nrf/src/main.c

@@ -191,5 +191,7 @@ int main(void)
 	printf("\nTest 8: Write block size API\n");
 	printf("   write-block-size = %u\n",
 	       flash_get_write_block_size(flash_dev));
+
+	printf("\nFinished!\n");
 	return 0;
 }