iommufd: Selftest coverage for IOMMU_IOAS_MAP_FILE

Add test cases to exercise IOMMU_IOAS_MAP_FILE.

Link: https://patch.msgid.link/r/1729861919-234514-10-git-send-email-steven.sistare@oracle.com


Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>