tools/testing/scatterlist: Test dynamic __sg_alloc_table_from_pages

Add few cases to test the dynamic allocation flow of
__sg_alloc_table_from_pages.

Link: https://lore.kernel.org/r/20201115120650.139277-1-leon@kernel.org


Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>