Send a PING frame at the start of QuartcPeer tests so that probing works.

Since QuartcPeer only sends datagram frames, it does not provide QUIC with any
retransmittable data to use for probes.  Sending a single PING frame at the
start of the connection (right after the handshake finishes) gives QUIC
something to retransmit for probing.

gfe-relnote: n/a (Quartc test only)
PiperOrigin-RevId: 244090790
Change-Id: Icc8a7fb0e4055bfc046283d2dfe7ca3c7133a686

quic/quartc/test/quartc_peer.cc

diff --git a/quic/quartc/test/quartc_peer.cc b/quic/quartc/test/quartc_peer.cc
index 036ba03..8aa1c1a 100644
--- a/quic/quartc/test/quartc_peer.cc
+++ b/quic/quartc/test/quartc_peer.cc
@@ -71,6 +71,7 @@
 
 void QuartcPeer::OnCryptoHandshakeComplete() {
   SetEnabled(true);
+  session_->SendPing();  // Hack to make probing work.
 }
 
 void QuartcPeer::OnConnectionWritable() {