test/py: Add cmd_memory dependency back to test_mmc_wr

Based on discussion with Stephen Warren there was recommendation to list
both memory and random command dependencies just in case that dependency is
not properly handled by Kconfig.

Fixes: a09c1f7e ("test/py: Fix MMC/SD block write test dependency")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Stephen Warren <swarren@nvidia.com>