memblock tests: Add memblock_reserve tests

Add checks for marking a region as reserved in different scenarios:
 - The region does not overlap with existing entries
 - The region overlaps with one of the previous entries: from the top
   (its end address is bigger than the base of the existing region) or
   from the bottom (its base address is smaller than the end address of
   one of the regions)
 - The region is within an already defined region
 - The same region is marked as reserved twice

Signed-off-by: Karolina Drobnik <karolinadrobnik@gmail.com>
Signed-off-by: Mike Rapoport <rppt@kernel.org>
Link: https://lore.kernel.org/r/cac867d2b6c17e53d9e977b5d6cd88cc4e9453b6.1643796665.git.karolinadrobnik@gmail.com