mmc_test: highmem tests

Add a couple of tests to make sure the host driver handles highmem
memory pages properly. Unfortunately there is no way to guarantee an
allocation below 4 GB in i386, so it might give you addresses that
are out of reach for the hardware (OTOH, so will any other highmem
allocation in the kernel).

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>