[Fix] test external_relay: count should always be the last rule,

as it have no matching condition

test/functional/configs/merged-local.conf

@@ -41,6 +41,9 @@ external_relay {
     }
     EXTERNAL_RELAY_COUNT {
       count = 4;
+      # `count` strategy always pops Received headers out, this will break other rules.
+      # So it should always be the last rule.
+      priority = 30;
       strategy = "count";
     }
     EXTERNAL_RELAY_HOSTNAME_MAP {