message: Fix tricky test
  This test was relying on the fact that a certain packet only had an ACK
  frame in it.  With the tweak to send CRYPTO frames more often, this was
  no longer the case.  I had to tweak it so that fewer packets were
  requested from the server.
diff: @@ -5500,18 +5500,17 @@ mod tests {
  let c_hs4 = client.process(s_hs3, now + (INCR * 3)).dgram();
  assert!(c_hs4.is_some()); // This will be acknowledged.
  - // Get an ACK for the client.
  + // Process c_hs2 and c_hs4, but skip c_hs3.
  + // Then get an ACK for the client.
  now += RTT / 2;
  - // Deliver the last one first, so that gets acknowledged.
  - // This won't generate an ACK, because it only contains an ACK.
  - let s_ack1 = server.process(c_hs4, now).dgram();
  - assert!(s_ack1.is_none());
  + // Deliver c_hs4 first, but don't generate a packet.
  + server.process_input(c_hs4.unwrap(), now);
  + let s_ack = server.process(c_hs2, now).dgram();
  + assert!(s_ack.is_some());
  // This includes an ACK, but it also includes HANDSHAKE_DONE,
  - // which we need to remove because that will cause the Handshake loss recovery
  - // state to be dropped.
  - let s_ack2 = server.process(c_hs2, now).dgram();
  - assert!(s_ack2.is_some());
  - let (s_hs_ack, _s_ap_ack) = split_datagram(s_ack2.unwrap());
  + // which we need to remove because that will cause the Handshake loss
  + // recovery state to be dropped.
  + let (s_hs_ack, _s_ap_ack) = split_datagram(s_ack.unwrap());
  // Now the client should start its loss recovery timer based on the ACK.
  now += RTT / 2;
  