siphash: Convert selftest to KUnit

Convert the siphash self-test to KUnit so it will be included in "all
KUnit tests" coverage, and can be run individually still:

$ ./tools/testing/kunit/kunit.py run siphash
...
[02:58:45] Starting KUnit Kernel (1/1)...
[02:58:45] ============================================================
[02:58:45] =================== siphash (1 subtest) ====================
[02:58:45] [PASSED] siphash_test
[02:58:45] ===================== [PASSED] siphash =====================
[02:58:45] ============================================================
[02:58:45] Testing complete. Ran 1 tests: passed: 1
[02:58:45] Elapsed time: 21.421s total, 4.306s configuring, 16.947s building, 0.148s running

Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: "Steven Rostedt (Google)" <rostedt@goodmis.org>
Cc: Yury Norov <yury.norov@gmail.com>
Cc: Sander Vanheule <sander@svanheule.net>
Acked-by: "Jason A. Donenfeld" <Jason@zx2c4.com>
Link: https://lore.kernel.org/lkml/CAHmME9r+9MPH6zk3Vn=buEMSbQiWMFryqqzerKarmjYk+tHLJA@mail.gmail.com
Tested-by: David Gow <davidgow@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>