dma-buf: Change st-dma-fence.c to use kunit

dma-buf: Change st-dma-fence.c to use kunit

Modernize the open coded test framework by using kunit.

Add a num_online_cpus() check to test_race_signal_callback() as the
default kunit.py runs the VM with a single CPU and this test depends on
two truly parallel kthreads. Skip it instead of hanging.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/2-v1-0a349a394eff+14110-dmabuf_kunit_jgg@nvidia.com