REGTESTS: explicitly use "balance roundrobin" where RR is needed

A few tests explicitly rely on the server ordering granted by
"balance roundrobin", but didn't specify the balance algorithm.
As it will change soon, let's explicit it.

reg-tests/sample_fetches/srv_name.vtc

@@ -31,6 +31,7 @@ haproxy h1 -conf {
         default_backend be
 
     backend be
+        balance roundrobin
         server srv1 ${s1_addr}:${s1_port}
         server srv2 ${s2_addr}:${s2_port}
 } -start

reg-tests/server/cli_delete_dynamic_server.vtc

@@ -36,6 +36,7 @@ haproxy h1 -conf {
 		default_backend test
 
 	backend test
+		balance roundrobin
 		server s1 ${s1_addr}:${s1_port}
 } -start