s_client exits on stdin EOF, and with TLS 1.3 the session ticket arrives after the
handshake. With </dev/null, the TLS 1.3 probe in get_server_certificate() misses the
ticket lifetime hint, and the -sess_out call in sub_session_resumption() writes no
session. Servers that issue tickets only over TLS 1.3 are then reported as "tickets no",
or intermittently as "connect problem". hold_stdin() keeps stdin open until the expected
output appears, for at most 2s. Fixes#3174.
