test: dm: add a test for class button

Add a test to confirm that we can read button state
using the button-gpio driver.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index 8391117..864c8d0 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -19,6 +19,7 @@
 obj-$(CONFIG_SOUND) += audio.o
 obj-$(CONFIG_BLK) += blk.o
 obj-$(CONFIG_BOARD) += board.o
+obj-$(CONFIG_BUTTON) += button.o
 obj-$(CONFIG_DM_BOOTCOUNT) += bootcount.o
 obj-$(CONFIG_CLK) += clk.o clk_ccf.o
 obj-$(CONFIG_DEVRES) += devres.o