selftests/iommu: fix the config fragment

The config fragment doesn't follow the correct format to enable those
config options which make the config options getting missed while
merging with other configs.

➜ merge_config.sh -m .config tools/testing/selftests/iommu/config
Using .config as base
Merging tools/testing/selftests/iommu/config
➜ make olddefconfig
.config:5295:warning: unexpected data: CONFIG_IOMMUFD
.config:5296:warning: unexpected data: CONFIG_IOMMUFD_TEST

While at it, add CONFIG_FAULT_INJECTION as well which is needed for
CONFIG_IOMMUFD_TEST. If CONFIG_FAULT_INJECTION isn't present in base
config (such as x86 defconfig), CONFIG_IOMMUFD_TEST doesn't get enabled.

Fixes: 57f09887

 ("iommufd: Add a selftest")
Link: https://lore.kernel.org/r/20240222074934.71380-1-usama.anjum@collabora.com
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>