test/py: test the md/mw commands

This tests whether md/mw work, and affect each-other.

Command repeat is also tested.

test/cmd_repeat.sh is removed, since the new Python-based test does
everything it used to.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>