test: mux-cmd: Add tests for the 'mux' command

Tests tests run the three mux subcommands: list, select, and deselect,
and verify that the commands do what we expect.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index 93484b4..8b3d77e 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -34,6 +34,7 @@
 obj-$(CONFIG_LED) += led.o
 obj-$(CONFIG_DM_MAILBOX) += mailbox.o
 obj-$(CONFIG_DM_MMC) += mmc.o
+obj-$(CONFIG_CMD_MUX) += mux-cmd.o
 obj-y += fdtdec.o
 obj-y += ofnode.o
 obj-y += ofread.o