test/py: mmc: Add 'mmc dev' test

Add separate test for 'mmc dev' subcommand. This tests whether
the system can switch to a specific card.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>