memblock tests: update tests to check if memblock_alloc zeroed memory

Add an assert in memblock_alloc() tests where allocation is expected to
occur. The assert checks whether the entire chunk of allocated memory is
cleared.

The current memblock_alloc() tests do not check whether the allocated
memory was zeroed. memblock_alloc() should zero the allocated memory since
it is a wrapper for memblock_alloc_try_nid().

Reviewed-by: Shaoqin Huang <shaoqin.huang@intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Rebecca Mckeever <remckee0@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/83ffb941b65074f40eb14552f8bfe5b71fe50abd.1661578349.git.remckee0@gmail.com