iommufd/selftest: Iterate idev_ids in mock_domain's alloc_hwpt test

The point in iterating variant->mock_domains is to test the idev_ids[0]
and idev_ids[1]. So use it instead of keeping testing idev_ids[0] only.

Link: https://lore.kernel.org/r/20230919011637.16483-1-nicolinc@nvidia.com


Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>