memblock tests: Add memblock_alloc_from tests for top down

Add checks for memblock_alloc_from for default allocation direction.
The tested scenarios are:
  - Not enough space to allocate memory at the minimal address
  - Minimal address parameter is smaller than the start address
    of the available memory
  - Minimal address is too close to the available memory

Add simple memblock_alloc_from test that can be used to test both
allocation directions (minimal address is aligned or misaligned).

Signed-off-by: Karolina Drobnik <karolinadrobnik@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/3dd645f437975fd393010b95b8faa85d2b86490a.1646055639.git.karolinadrobnik@gmail.com