selftests: netfilter: prefer xfail in case race wasn't triggered

selftests: netfilter: prefer xfail in case race wasn't triggered

Jakub says: "We try to reserve SKIP for tests skipped because tool is
missing in env, something isn't built into the kernel etc."

use xfail, we can't force the race condition to appear at will
so its expected that the test 'fails' occasionally.

Fixes: 78a588363587 ("selftests: netfilter: add conntrack clash resolution test case")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20251206175647.5c32f419@kernel.org/
Signed-off-by: Florian Westphal <fw@strlen.de>