Add AuTest for connect_attempts_max_retries_down_server

[masaori335]

This is an effort to verify connect_attempts_max_retries_down_server behavior.

The doc for proxy.config.http.connect_attempts_max_retries_down_server says below.

Maximum number of connection attempts Traffic Server can make while an origin is marked down per request.

Also doc of proxy.config.http.connect_attempts_max_retries says below

Once the maximum number of retries is reached, the origin is marked down. After this, the setting proxy.config.http.connect_attempts_max_retries_down_server is used to limit the number of retry attempts to the known down origin.

However, I don't see ATS makes connection attempts against down servers. From my understanding, HostDBRecord::select_best_http doesn't return HostDBInfo which is in down state. How HttpSM can try to down servers?