REGTESTS: sample_fetches: do not race with the server close in proxy_state

REGTESTS: sample_fetches: do not race with the server close in proxy_state

A full run of the suite caught srv_conn() reporting zero where the test
expects one. The server closed its connection as soon as it had
answered, and when that close reached haproxy before the response rules
were evaluated, the server connection was already released and the
counter had gone back to zero.

The server now holds its connection until the client has read the
response, which is necessarily after the rules have run.