iommufd/selftest: Add domain_alloc_user() support in iommu mock

Add mock_domain_alloc_user() and a new test case for
IOMMU_HWPT_ALLOC_NEST_PARENT.

Link: https://lore.kernel.org/r/20230928071528.26258-6-yi.l.liu@intel.com


Co-developed-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>