memblock tests: Add memblock_alloc tests for top down

Add checks for memblock_alloc for top down allocation direction.
The tested scenarios are:
  - Region can be allocated on the first fit (with and without
    region merging)
  - Region can be allocated on the second fit (with and without
    region merging)

Add checks for both allocation directions:
  - Region can be allocated between two already existing entries
  - Limited memory available
  - All memory is reserved
  - No available memory registered with memblock

Signed-off-by: Karolina Drobnik <karolinadrobnik@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/26ccf409b8ff0394559d38d792b2afb24b55887c.1646055639.git.karolinadrobnik@gmail.com