memblock tests: update style of comments for memblock_add_*() functions

Update comments in memblock_add_*() functions to match the style used
in tests/alloc_*.c by rewording to make the expected outcome more apparent
and, if more than one memblock is involved, adding a visual of the
memory blocks.

Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Rebecca Mckeever <remckee0@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>