Add proper parse logic for the SSv2 SentMessage format

This isn't hooked up to anything that the fan_out test method wasn't,
but it will come in handy for future testing.

rust/protocol/tests/support/mod.rs

@@ -196,6 +196,16 @@ pub fn initialize_sessions_v4() -> Result<(SessionRecord, SessionRecord), Signal
     Ok((alice_session, bob_session))
 }
 
+pub fn extract_single_ssv2_received_message(input: &[u8]) -> (ServiceId, Vec<u8>) {
+    let message = SealedSenderV2SentMessage::parse(input).expect("valid");
+    assert_eq!(1, message.recipients.len());
+    let result = message
+        .received_message_parts_for_recipient(&message.recipients[0])
+        .as_ref()
+        .concat();
+    (message.recipients[0].service_id, result)
+}
+
 pub enum IdChoice {
     Exactly(u32),
     Next,