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

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Good conversion overall. The `num_online_cpus() < 2` skip in `test_race_signal_callback` is the right fix for the kunit.py single-CPU VM environment.

**Typo:** In `test_wait_timeout`:
```c
			kunit_mark_skipped(
				test, "Timer did not fire within on HZ!\n");
```
"within **on** HZ" should be "within **one** HZ". Also, the trailing `\n` is unnecessary in kunit messages.

**`dma_fence_suite_init` is arguably unnecessary:** The function only prints `sizeof(dma_fence)`:
```c
static int dma_fence_suite_init(struct kunit_suite *suite)
{
	pr_info("sizeof(dma_fence)=%zu\n", sizeof(struct dma_fence));
	return 0;
}
```
This doesn't set up any resources that the tests depend on. It's a minor informational message. Not wrong, but `suite_init` is normally used for resource initialization (as done correctly in patch 4 for the slab cache). If the pr_info is desired, it could also be placed in a test or simply dropped.

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