vfio: selftests: Replicate tests across all iommu_modes

vfio: selftests: Replicate tests across all iommu_modes

Automatically replicate vfio_dma_mapping_test and vfio_pci_driver_test
across all supported IOMMU modes using fixture variants. Both of these
tests exercise DMA mapping to some degree so having automatic coverage
across all IOMMU modes will help catch bugs.

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20250822212518.4156428-26-dmatlack@google.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>