memblock tests: Add memblock_alloc tests for bottom up

Add checks for memblock_alloc for bottom up 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 test case wrappers to test both directions in the same context.

Signed-off-by: Karolina Drobnik <karolinadrobnik@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/426674eee20d99dca49caf1ee0142a83dccbc98d.1646055639.git.karolinadrobnik@gmail.com