arm: Add memtest support

Add a test to touch all memory allocated to the guest.
Provides options to allocate in block, shared mode etc.
Also adds a "memstress" variant which would test all the
combinations in order.

PS: The memory allocator fragments the available memory
on page allocation and doesn't allow merging them for a
higher order allocation. Hence, all the block alloc tests
are run one after the other, before any page allocation
tests are run

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Co-developed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>