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>

test/dm/Makefile

@@ -34,6 +34,7 @@ obj-y += irq.o
 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