server_add() only waits for CQL readiness before returning. The
Alternator HTTP port may not be listening yet, causing
ConnectionRefused with Alternator tests.
Extend the ServerUpState enum and startup loop to also check Alternator
port readiness when configured. Whenever Alternator port(s) is/are
configured, each is verified if connectable and queryable,
similar to how CQL ports are probed.
Fixes SCYLLADB-1701
Closesscylladb/scylladb#29625
d5efd1f676