REGTESTS: check attach-srv out of order declaration

Previous patch fixed a regression which caused some config with
attach-srv to be rejected if the rule was declared before the target
server itself. To better detect this kind of error, mix the declaration
order in the corresponding regtest.

reg-tests/connection/reverse_connect_full.vtc

@@ -23,12 +23,12 @@ frontend pub
 	bind "fd@${pub}"
 	use_backend be-reverse
 
-backend be-reverse
-	server dev rhttp@
-
 frontend priv
 	bind "fd@${priv}" proto h2
 	tcp-request session attach-srv be-reverse/dev
+
+backend be-reverse
+	server dev rhttp@
 } -start
 
 haproxy h_dev -conf {