kasan: change kasan_atomics kunit test as KUNIT_CASE_SLOW

During running KASAN Kunit tests with CONFIG_KASAN enabled, the following
"warning" is reported by kunit framework:

	# kasan_atomics: Test should be marked slow (runtime: 2.604703115s)

It took 2.6 seconds on my PC (Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz),
apparently, due to multiple atomic checks in kasan_atomics_helper().

Let's mark it with KUNIT_CASE_SLOW which reports now as:

	# kasan_atomics.speed: slow

Link: https://lkml.kernel.org/r/20241101184011.3369247-3-snovitoll@gmail.com
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: novitoll

mm/kasan/kasan_test_c.c

@@ -2075,7 +2075,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
 	KUNIT_CASE(kasan_strings),
 	KUNIT_CASE(kasan_bitops_generic),
 	KUNIT_CASE(kasan_bitops_tags),
-	KUNIT_CASE(kasan_atomics),
+	KUNIT_CASE_SLOW(kasan_atomics),
 	KUNIT_CASE(vmalloc_helpers_tags),
 	KUNIT_CASE(vmalloc_oob),
 	KUNIT_CASE(vmap_tags),