test(client): update test helpers to handle fallible packet length calculations

crates/irtt-client/tests/large_packets.rs

@@ -1,10 +1,12 @@
 mod support;
 
 use irtt_client::{Client, ClientConfig, ClientEvent, NegotiationPolicy};
-use irtt_proto::{echo_header_len, echo_packet_len, Params, TimestampFields};
+use irtt_proto::{echo_header_len, Params, TimestampFields};
 
 use support::{config_for_params, default_params, run_one_probe, BackendPeer, ServerObservation};
 
+use crate::support::test_echo_packet_len;
+
 fn params_with_length(length: i64) -> Params {
     Params {
         length,