memblock: test suite improvements

* Added verification that memblock allocations zero the allocated memory
* Added more test cases for memblock_add(), memblock_remove(),
  memblock_reserve() and memblock_free()
* Added tests for memblock_*_raw() family
* Added tests for NUMA-aware allocations in memblock_alloc_try_nid() and
  memblock_alloc_try_nid_raw()
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