iommufd/selftest: Skip when the mock device is not there

Both iommufd tests need the mock device the iommufd module registers
under CONFIG_IOMMUFD_TEST, and they open /dev/iommu in every fixture
without checking, so on a kernel that does not provide it every case
fails on the open.

Checking the open is not enough on its own. /dev/iommu appears as soon
as the iommufd module is loaded, which says nothing about the mock
device: the release config here is CONFIG_IOMMUFD=m with
CONFIG_IOMMUFD_TEST unset, so the open succeeds and the tests fail later
in the ioctls that drive the mock device instead. Only the debug config
sets CONFIG_IOMMUFD_TEST.

/sys/bus/iommufd_mock covers both, it exists only once the module is
loaded and the kernel was built with CONFIG_IOMMUFD_TEST. Check for it
once before handing over to the harness, rather than in each of the
fixtures, and report a skip. test_harness_run() is what TEST_HARNESS_MAIN
calls anyway, so the tests still run exactly as they did.

Signed-off-by: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
Link: https://patch.msgid.link/20260908214825.2682078-2-eva.kurchatova@virtuozzo.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

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