test/py: add MMC/SD block write test

Add a standalone MMC block write test. This allows direct testing of MMC
access rather than relying on doing so as a side-effect of e.g. DFU or
UMS testing, which may not be enabled on all platforms.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>