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: 78a5883 ("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>

Author: Florian Westphal

tools/testing/selftests/net/netfilter/conntrack_clash.sh

@@ -116,7 +116,7 @@ run_one_clash_test()
 	# not a failure: clash resolution logic did not trigger.
 	# With right timing, xmit completed sequentially and
 	# no parallel insertion occurs.
-	return $ksft_skip
+	return $ksft_xfail
 }
 
 run_clash_test()
@@ -133,12 +133,12 @@ run_clash_test()
 		if [ $rv -eq 0 ];then
 			echo "PASS: clash resolution test for $daddr:$dport on attempt $i"
 			return 0
-		elif [ $rv -eq $ksft_skip ]; then
+		elif [ $rv -eq $ksft_xfail ]; then
 			softerr=1
 		fi
 	done
 
-	[ $softerr -eq 1 ] && echo "SKIP: clash resolution for $daddr:$dport did not trigger"
+	[ $softerr -eq 1 ] && echo "XFAIL: clash resolution for $daddr:$dport did not trigger"
 }
 
 ip link add veth0 netns "$nsclient1" type veth peer name veth0 netns "$nsrouter"
@@ -167,8 +167,7 @@ load_simple_ruleset "$nsclient2"
 run_clash_test "$nsclient2" "$nsclient2" 127.0.0.1 9001
 
 if [ $clash_resolution_active -eq 0 ];then
-	[ "$ret" -eq 0 ] && ret=$ksft_skip
-	echo "SKIP: Clash resolution did not trigger"
+	[ "$ret" -eq 0 ] && ret=$ksft_xfail
 fi
 
 exit $ret