[nrf fromtree] tests: Bluetooth: Mesh: Use flash_area_flatten for erase

The flash_area_erase is replaced with flash_area_flatten that
is also able to erase/scramble devices that do not require
explicit call to erase procedure before write.

Signed-off-by: Dominik Ermel <[email protected]>
(cherry picked from commit 8dc1fce)

Author: de-nordic

tests/bluetooth/mesh/blob_io_flash/src/main.c

@@ -68,7 +68,7 @@ ZTEST(blob_io_flash, test_chunk_read)
 	err = flash_area_open(SLOT1_PARTITION_ID, &fa);
 	zassert_equal(err, 0, "Preparing test data failed with err=%d", err);
 
-	err = flash_area_erase(fa, 0, ARRAY_SIZE(ctrl_data));
+	err = flash_area_flatten(fa, 0, ARRAY_SIZE(ctrl_data));
 	zassert_equal(err, 0, "Preparing test data failed with err=%d", err);
 
 	err = flash_area_write(fa, 0, test_data, ARRAY_SIZE(ctrl_data));