REGTESTS: sample_fetches: add a test for the proxy and server state fetches

REGTESTS: sample_fetches: add a test for the proxy and server state fetches

None of the fetches reporting the internal state of the proxies and of
their servers was covered. This test checks fe_id, fe_name, fe_defbe,
be_id, be_name, srv_id, srv_name, nbsrv with and without an argument,
srv_is_up, srv_weight, srv_iweight, srv_uweight, act_conn, fe_conn,
be_conn, be_conn_free, srv_conn, srv_conn_free, connslots, queue,
avg_queue, srv_queue, fe_sess_rate, fe_req_rate, be_sess_rate,
srv_sess_rate, stopping, uptime and fe_client_timeout, plus the nbsrv,
srv_is_up, srv_queue and fe_exists converters.

The backend holds a usable server and a disabled one so that nbsrv() and
srv_is_up() have something to distinguish, and both servers have a
maxconn and a maxqueue so that connslots() returns a computable value
instead of the -1 it reports on an unbounded configuration.

The fetches whose implicit argument is the current proxy are placed in
the backend since they require the backend capability.