iommufd: Add config needed for iommufd_fail_nth

Add FAULT_INJECTION_DEBUG_FS and FAILSLAB configurations to the kconfig
fragment for the iommfd selftests. These kconfigs are needed by the
iommufd_fail_nth test.

Fixes: a9af47e3 ("iommufd/selftest: Test IOMMU_HWPT_GET_DIRTY_BITMAP")
Link: https://lore.kernel.org/r/20240325090048.1423908-1-usama.anjum@collabora.com


Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>