doc: driver-model: Update dm tests run using test.py

Since all the tests are implemented in pytest infrastructure,
So update the dm tests with the same 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>