tests: socket: tls: check sendmsg() behavior for datagrams

Datagrams (for DTLS connection) need to be sent using single API call to
mbedTLS and should not result in sending each fragment in a separate
datagram. Check if that is the case, so allow only single fragment data to
be sent and expect an error when multiple fragments were passed to
sendmsg().

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>