[nrf fromtree] tests/dfu: Replace flash_area_erase with flash_area_flatten

Use flash_area_flatten instead of flash_area_erase; this allows
to run tests on devices that do not require explicit erase
before write or do not provide the callback.

Signed-off-by: Dominik Ermel <[email protected]>
(cherry picked from commit 7b36d34)

Author: de-nordic

tests/subsys/dfu/img_util/src/main.c

@@ -73,7 +73,7 @@ ZTEST(img_util, test_collecting)
 			       sizeof(erase_buf));
 	zassert_true(ret == 0, "Flash write failure (%d)", ret);
 #else
-	ret = flash_area_erase(ctx.flash_area, 0, ctx.flash_area->fa_size);
+	ret = flash_area_flatten(ctx.flash_area, 0, ctx.flash_area->fa_size);
 	zassert_true(ret == 0, "Flash erase failure (%d)", ret);
 #endif
 
@@ -137,7 +137,7 @@ ZTEST(img_util, test_check_flash)
 
 	ret = flash_img_init_id(&ctx, SLOT1_PARTITION_ID);
 	zassert_true(ret == 0, "Flash img init 1");
-	ret = flash_area_erase(ctx.flash_area, 0, ctx.flash_area->fa_size);
+	ret = flash_area_flatten(ctx.flash_area, 0, ctx.flash_area->fa_size);
 	zassert_true(ret == 0, "Flash erase failure (%d)\n", ret);
 	ret = flash_img_buffered_write(&ctx, tst_vec, sizeof(tst_vec), true);
 	zassert_true(ret == 0, "Flash img buffered write\n");