test: overlay: Add unit test for stacked overlay

Verify that stacked overlays work.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/test/overlay/Makefile b/test/overlay/Makefile
index 907f085..416645c 100644
--- a/test/overlay/Makefile
+++ b/test/overlay/Makefile
@@ -13,3 +13,4 @@
 # DT overlays
 obj-y += test-fdt-base.dtb.o
 obj-y += test-fdt-overlay.dtb.o
+obj-y += test-fdt-overlay-stacked.dtb.o