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

Update comments in memblock_reserve_*() 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.

If the comment has an extra column of spaces, remove the extra space at
the beginning of each line for consistency and to conform to Linux kernel
coding style.

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