uboot-imx

board: README.sandbox: Update dm test command

Update dm test command with pytest instead of ./test/dm/test-dm.sh

Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>